USMLE Exam 2

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Kubler-Ross’s Death and Dying sequence is a stepwise process with five identified stages, which occur in the following order: 1) Denial, 2) Anger, 3) Bargaining, 4) Sadness, and 5) Acceptance. "Doctor you must be wrong" is the correct answer since it reflects the patient's inability to accept the information and indicates the denial of the first stage. "Can you keep me alive until my daughter graduates from medical school", is a statement from the third, or bargaining, stage. "Damn you doctor, you should have caught this earlier" is a statement from the second, or anger, phase. "I think it is time that I make a will and say good-bye to everyone" reflects the patient's acceptance of the reality and is a statement from the fifth phase (acceptance). "It's no use, I always lose and get the short end of the stick" is a statement from the fourth phase (sadness).

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Rupture of the free left ventricular wall is a frequently fatal complication that may occur in the first week after myocardial infarction (MI). At this stage, the infarcted area is composed of friable necrotic myocardium and early granulation tissue. It is during this crucial phase, therefore, that rupture usually occurs. Blood rushes out, filling the pericardial sac and causing compression of the left ventricle. Cardiac tamponade ensues, and the patient usually dies of acute cardiogenic shock. Aortic dissection is not a complication of MI, although cardiac tamponade may also follow this acute condition when dissection works its way back toward the aortic root. Aortic dissection usually develops in aortas affected by cystic medial degeneration (CMD), which is due to fragmentation of elastic laminae with accumulation of myxoid material in the aortic media. CMD may be either sporadic or associated with Marfan syndrome. Extension of a previous MI may occur in the first few hours or days after MI. It may aggravate or precipitate cardiogenic shock and/or arrhythmias, but it does not cause cardiac tamponade. Arrhythmias are frequent complications of MI and are often fatal, producing cardiac arrest (ventricular fibrillation) or aggravating cardiac dysfunction. If infarction involves papillary muscles, these may rupture. This complication is followed by valvular dysfunction and may manifest with signs of mitral regurgitation and acute congestive heart failure.

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Tumors of the ventricular system of the brain can affect the brain tissue either directly, via pressure on or invasion into a physically close structure, or indirectly, by obstructing CSF flow and causing hydrocephalus. Choroid plexus papillomas are highly differentiated, benign tumors that can involve any ventricle but have a predilection for involving the lateral ventricles of small children, particularly boys. The caudate nucleus is a C-shaped structure that comprises part of the wall of the lateral ventricle throughout its extent. The only structure listed that is adjacent to the body of the lateral ventricle, and would therefore be directly affected by the large tumor described in the question, is the caudate nucleus. The cerebellum overlies the fourth ventricle. The hippocampus is adjacent to the inferior (temporal) horn of the lateral ventricle. The hypothalamus abuts the third ventricle. The pons forms part of the floor of the fourth ventricle.

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The left atrium forms most of the posterior wall of the heart. The esophagus passes immediately posterior to the heart. Enlargement of the left atrium may compress the esophagus and cause dysphagia. The left ventricle forms most of the left border of the heart and most of the diaphragmatic surface of the heart. The left ventricle is not related to the esophagus. The pulmonary trunk emerges from the right ventricle on the anterior surface of the heart. The pulmonary trunk is not related to the esophagus. The right atrium forms the right border of the heart. It is not related to the esophagus. The right ventricle forms most of the anterior wall of the heart and a small portion of the diaphragmatic surface of the heart. It is not related to the esophagus.

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The single finding of high autoantibody titers to histones, without any other autoantibodies, is characteristic of drug-induced lupus. The most commonly implicated drugs are procainamide, hydralazine (given for hypertension), and isoniazid. Patients typically have milder disease than in systemic lupus erythematosus (SLE) and tend to have arthritis, pleuropericardial involvement, and, less commonly, rash. CNS and renal disease are not usually observed. CREST syndrome is a milder variant of scleroderma characterized by calcinosis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly and telangiectasia. Anti-centromere antibodies are diagnostic. The diffuse form of scleroderma, also known as systemic sclerosis, causes fibrosis of the skin and internal viscera. This disorder is characterized by anti-SCI-70 and often low titers of many other autoantibodies. Sjögren syndrome is characterized by dry eyes and dry mouth. Sjögren syndrome in isolation is characteristically positive for anti-SS-A and anti-SS-B. If it accompanies rheumatoid arthritis, anti-RNP will be positive as well. SLE is a multisystem disorder that is distinguished from drug-induced lupus by the presence of a wide variety of autoantibodies, including anti-double stranded DNA (Anti dsDNA).

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Sarcoidosis is a multisystem disease characterized by noncaseating granulomata in a variety of organs. The disease may be symptomatic (respiratory and constitutional symptoms) or may be discovered incidentally when chest x-ray or autopsy reveals bilateral hilar adenopathy. Definitive diagnosis is based on biopsy, which reveals noncaseating granulomata that are negative for fungi or acid-fast bacilli. Sarcoidosis is more common in individuals of African-American descent. Caroli disease is a congenital malformation of the bile duct system. Raynaud disease is a vasospasm of vessels that causes temporary ischemia in the hands. Scleroderma, or progressive systemic sclerosis, is characterized by progressive fibrosis of skin and internal organs. Systemic lupus erythematosus is an autoimmune disease characterized by vasculitis (which may produce a variety of symptoms depending on the site of the lesion), rash, renal disease, hemolytic anemia, and neurologic disturbances.

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The patient has gout, which is due to precipitation of monosodium urate crystals in joint spaces (notably the great toe) and soft tissues (causing tophi, which are often found on the external ears). Colchicine reduces the inflammation caused by the urate crystals by inhibiting leukocyte migration and phagocytosis secondary to an effect on microtubule assembly. Allopurinol and its metabolite, oxipurinol, inhibit xanthine oxidase, the enzyme that forms uric acid from hypoxanthine. Therapy with this agent should be begun 1-2 weeks after the acute attack has subsided. Aspirin competes with uric acid for tubular secretion, thereby decreasing urinary urate excretion and raising serum uric acid levels. At high doses (more than 2 gm daily) aspirin is a uricosuric. Probenecid and sulfinpyrazone are uricosuric agents, increasing the urinary excretion of uric acid, hence decreasing serum levels of the substance. Therapy with these agents should be begun 1-2 weeks after the acute attack has subsided.

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The tumor described is a craniopharyngioma, which is one of the more common brain tumors of children. These tumors arise from epithelial rests derived from Rathke’s pouch, which is an oral invagination that gives rise to the cells that form the anterior pituitary gland. Histologically, craniopharyngiomas can resemble ameloblastomas, which are tumors derived from dental epithelium. Note that this question could also have been answered very simply by noting that the hypothalamus, pineal gland, pituitary gland, and the superior colliculus are all adult structures; only Rathke’s pouch is an embryonic structure. Therefore, only Rathke’s pouch could be the source of epithelial rests, which are remnants of embryonic tissues that persist in the adult. Although craniopharyngiomas often occur near the hypothalamus and posterior portion of the pituitary gland, they do not arise from neural tissue. The pineal gland and superior colliculus are found on the posterior aspect of the brain stem.

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A patient with aortic regurgitation, caused by insufficiency of the aortic valve, has a wide pulse pressure (the difference between systolic and diastolic pressure). In fact, during diastole, the systemic pressure precipitously drops as the blood flows from the aorta back into the left ventricle through the incompetent aortic valve. Systolic pressure remains relatively normal since it depends on the left ventricular ejection. Aortic stenosis is associated with reduced systolic pressure and relatively preserved diastolic pressure, such as 95/80 mm Hg, since the left ventricle is unable to pump a normal amount of blood through a stenotic valvular orifice. A blood pressure of 50/undetectable mm Hg is characteristic of acute shock. A blood pressure of 120/80 mm Hg is considered within normal limits in healthy adults, whereas 160/95 mm Hg is definitely in the range of hypertension, although mild. A blood pressure of 220/130 mm Hg is typical of malignant hypertension, a severe condition that may lead to life-threatening complications if not promptly treated.

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Nodular lymphomas of all types are derived from the B-cell line. The translocation t(14, 18), with bcl-2 activation, is associated with these lymphomas. An abl-bcr hybrid (bcr-c-abl;) and t(9, 22) translocation are associated with chronic myeloid leukemia (CML). c-myc activation and t(8, 14) are associated with Burkitt lymphoma.

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At this level, the lateral portion of the dorsal columns (funiculus) is composed of the fasciculus cuneatus. Axons carrying tactile, proprioceptive, and vibratory information from the ipsilateral arm enter the spinal cord via the dorsal root, ascend the cord in the fasciculus cuneatus, and synapse in the nucleus cuneatus of the caudal medulla. Secondary neurons from this nucleus give rise to internal arcuate fibers, which decussate and ascend to the thalamus (ventral posterolateral nucleus, VPL) as the medial lemniscus. Tertiary neurons from the VPL project to the ipsilateral somatosensory cortex. Therefore, damage to the fasciculus cuneatus would result in a deficit in tactile, proprioceptive, and vibratory sense in the ipsilateral arm, because the fibers that carry this information do not cross until they reach the medulla. Fine motor control of the fingers would be carried principally by the ipsilateral lateral corticospinal tract in the lateral funiculus of the cord. Motor control of the contralateral foot is carried by the ipsilateral corticospinal tract in the lateral funiculus of the cord. Proprioception from the ipsilateral leg is carried by the fasciculus gracilis in the medial part of the dorsal columns. Hemianhidrosis (lack of sweating) of the face could be produced by interruption of sympathetic innervation to the face. The hypothalamospinal tract projects from the hypothalamus to the intermediolateral cell column at levels T-1 to T-2. It descends the cord in the lateral funiculus of the cord. Interruption of this tract results in Horner syndrome (miosis, ptosis, hemianhidrosis).

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The recurrent laryngeal nerves are branches of the vagus (CN X) and supply all intrinsic muscles of the larynx except the cricothyroid. The right recurrent laryngeal nerve recurs around the right subclavian artery. The left recurrent laryngeal nerve recurs in the thorax around the arch of the aorta and ligamentum arteriosum. Both nerves ascend to the larynx by passing between the trachea and esophagus, near the thyroid gland. The recurrent laryngeal nerves are therefore particularly vulnerable during thyroid surgery, and damage may cause extreme hoarseness. The facial nerve innervates the muscles of facial expression, the stapedius muscle, and the lacrimal, submandibular and sublingual glands. It also mediates taste sensation from the anterior two thirds of the tongue. The glossopharyngeal nerve innervates the stylopharyngeus muscle and the parotid gland. Visceral afferents supply the carotid sinus baroreceptors and carotid body chemoreceptors, and mediate taste from the posterior one third of the tongue. Somatosensory fibers supply pain, temperature, and touch information from the posterior one third of the tongue, upper pharynx, middle ear, and eustachian tube. The hypoglossal nerve innervates the intrinsic muscles of the tongue, the genioglossus, hypoglossus, and styloglossus muscles. The trigeminal nerve receives sensory information from the face and innervates the muscles of mastication.

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The child has neonatal respiratory distress syndrome (hyaline membrane disease). This condition is caused by the inability of the immature lungs to synthesize adequate amounts of surfactant. Surfactant, which reduces surface tension, helps keep alveoli dry, and aids in expansion of the lungs, is synthesized by type II pneumocytes. Alveolar capillary endothelial cells are important in maintaining the capillary structure and permitting flow of gases into and out of the bloodstream. Bronchial mucous cells produce the usually thin (in healthy individuals) coat of mucus that lines the bronchi. The ciliated bronchial respiratory epithelium is responsible for moving the dust-coated mucus layer out of the bronchi. Type I pneumocytes are the squamous cells that line alveoli and permit easy gas exchange. These cells tend to be immature (and thick) in premature infants but are not the producers of surfactant.

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The smell of acetone on the breath of a comatose patient is an important, rapid diagnostic clue that strongly suggests ketoacidosis and is usually seen in patients with poorly controlled type 1 diabetes. Other features of diabetic ketoacidosis include high blood glucose, increased serum osmolality, hypovolemia, acidosis, and electrolyte imbalance. In alcohol intoxication, the breath will smell like alcohol. Diabetic hyperosmolar coma usually is seen in older patients with type 2 diabetes and is not characterized by ketoacidosis. Since there is no acetone production, there is no specific scent to the breath. In heroin overdose, no acetone production occurs and there is no specific scent to the breath. In hypoglycemic coma, which can occur in diabetics with insulin overdose, no acetone production occurs and there is no specific scent to the breath.

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You should recognize this as a probable case of multiple endocrine neoplasia, specifically, MEN III (formerly MEN II b). Features of this autosomal dominant condition include medullary carcinoma of the thyroid, pheochromocytoma, and oral and intestinal ganglioneuromatosis (including mucosal neuromas). Gastrinomas, insulinomas, and pituitary adenomas are found in MEN I. Parathyroid adenomas are found in MEN I and II.

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The child likely has cystic fibrosis. In this disorder, an abnormality of chloride channels causes all exocrine secretions to be more viscous than normal. Pancreatic secretion of digestive enzymes is often severely impaired, with consequent steatorrhea and deficiency of fat-soluble vitamins, including vitamin A. Cystinuria is a relatively common disorder in which a defective transporter for dibasic amino acids (cystine, ornithine, lysine, arginine; COLA) leads to saturation of the urine with cystine, which is not very soluble in urine, and precipitates out to form stones. Hypoglycemia is not a prominent feature of children with cystic fibrosis who are on a normal diet. Hyperglycemia may occur late in the course of the disease. Iron deficiency anemia is not typically found in children with cystic fibrosis. Sphingomyelin accumulation is generally associated with deficiency of sphingomyelinase, as seen in Niemann-Pick disease.

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There are two main morphologic forms of emphysema, centriacinar and panacinar. The panacinar variant is related to alpha1-antitrypsin deficiency; the entire acinus is enlarged, from the respiratory bronchiole to the distal alveoli. Centriacinar emphysema is characterized by enlargement of the central portions of the acinus, i.e., the respiratory bronchiole, and its pathogenesis is related to exposure to tobacco products and coal dust. Interstitial emphysema is not a true form of emphysema. It results from penetration of air into the pulmonary interstitium. This may occur when alveolar tears develop because of a combination of coughing and airway obstruction (e.g., children with whooping cough) or a chest wound that injures the underlying lung parenchyma (e.g., a fractured rib). Compensatory emphysema and paraseptal emphysema are associated with scarring. Both are frequent but usually clinically silent. Paraseptal emphysema, however, may lead to spontaneous pneumothorax in young patients. In fact, this form is more severe in areas adjacent to the pleura, where large, cyst-like structures may develop and rupture into the pleural cavity.

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Urinary tract infections are the most common bacterial infections encountered during pregnancy, and Escherichia coli is the most commonly isolated organism. In the U.S., 70% of cases are caused by P pili-positive strains. Bundle-forming pili are found in enteroaggregative E. coli (EAEC). GVVPQ fimbriae are found in EAEC. Heat labile toxins are pathogenic factors in enterotoxic strains (ETEC). Heat stable toxins are pathogenic factors in ETEC or EAEC. Type 1 pili are a major pathogenic factor in ETEC.

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The tumor is hepatocellular carcinoma, which usually develops in the setting of cirrhosis owing to a variety of damaging agents, including hepatitis B virus (HBV) infection, alcohol use, and hemachromatosis. Epstein-Barr virus (EBV) is associated with Burkitt lymphoma and nasopharyngeal carcinoma. Human herpesvirus type 8 (HHV 8), a member of the herpes family, is associated with Kaposi sarcoma. Human papillomavirus (HPV) is associated with cervical, penile, and anal carcinoma. Human T-lymphocyte virus (HTLV-1) is associated with adult T-cell leukemia.

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This woman has dementia of the Alzheimer type. A gradual onset of symptoms, general pervasive memory deficit, difficulties with language, and inability to plan, leading to severe impairment of daily functioning, are all characteristic of this dementia. Amnestic disorder is limited to memory problems, and this woman is demonstrating cognitive dysfunction, such as alterations in language and the loss of the ability to plan. Pseudodementia is incorrect since it is a major depressive disorder rather than a dementing condition. There is no evidence for a depressive syndrome in this patient’s presentation. The diagnosis of substance-induced persisting dementia requires evidence of a history of substance abuse. However, it is the second most likely diagnosis and should be carefully explored with the husband and other close relatives and friends. Vascular dementia is generally characterized by a stepwise deterioration, not the gradual presentation of this case.

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This patient has an aortic dissection (formerly called dissecting aneurysm), a potentially fatal condition that is too often confused clinically with myocardial infarction. The most important clinical clue is that the pain shifts with time. Noninvasive techniques, such as transesophageal echocardiography, CT, and MRI, are increasingly useful in making this diagnosis. Aortic valve stenosis would not be expected to produce severe chest pain of acute onset. This patient’s clinical history does not suggest either an atherosclerotic or a syphilitic aneurysm. Even if he had one of either of these types of aneurysms and it had begun to rupture, the distinctive feature of severe pain moving downward would probably not be present. Myocardial infarction is the major diagnosis most often confused with this patient’s condition. The movement of the pain is the major clinical tip-off suggesting that this is not the correct answer.

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This is a psychotic level disorder (the patient is hallucinating). The patient has a fluctuating level of consciousness and is disoriented. Also, there is a clear history of a febrile condition that developed rather rapidly, all of which suggest delirium. In acute stress disorder a traumatic event occurs that precipitates an anxiety-type reaction, not a change in the sensorium. In both bipolar I disorder, manic type and brief psychotic disorder, patients may reach a level of behavioral disruption of psychotic proportion. They do not, however, demonstrate changes in level of consciousness or major disorientation. Persons with dementia demonstrate a clear sensorium with no fluctuations in the level of consciousness. In addition, persons with dementia predominantly show symptoms of impairment of cognitive functions (e.g., memory impairment).

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The right ventricle forms most of the anterior wall of the heart and extends from approximately the right border of the sternum to approximately 2 inches to the left of the sternum at the level of the fourth intercostal space. The left atrium forms the posterior wall of the heart. The only portion of the left atrium seen on the anterior surface of the heart is the left auricular appendage, which is at the level of the second intercostal space on the left. The left ventricle forms most of the left border of the heart and the diaphragmatic surface of the heart. It forms the anterior wall of the heart in a region from approximately 2-3 inches from the left border of the sternum from the third to the fifth intercostal space. The right atrium forms the right border of the heart. Its anterior surface is on the right side of the sternum from approximately the third rib to the sixth rib. The left lung is displaced away from the sternum on the left side by the presence of the heart.

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The patient has tabes dorsalis, which is a form of neurosyphilis seen 10 to 25 years or longer after primary disease. The pupils described are Argyll Robertson pupils and are considered diagnostic for neurosyphilis. Characteristically, the dorsal columns, which contain the ascending tracts for sensory information, become atrophic, probably as a result of damage to the dorsal root ganglion cells. The dorsal gray horn contains neurons that respond to sensory input. The lateral column contains both descending (e.g., lateral corticospinal, rubrospinal tracts) and ascending (e.g., spinocerebellar tracts, spinothalamic tracts) tracts. The ventral column contains both descending (e.g., anterior corticospinal, tectospinal) and ascending (e.g., spinothalamic) tracts. The ventral horn contains lower motor neurons.

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A right dominant coronary circulation is present when the posterior descending branch originates from the right coronary artery (80% of individuals). On the contrary, the posterior descending artery originates from the left circumflex artery in a left dominant circulation (20% of individuals). The posterior descending branch gives blood to the posterior half of the interventricular septum. Occlusion of the left circumflex artery in a left dominant circulation will therefore lead to ischemic necrosis in the left ventricular wall and the posterior interventricular septum. The apex of the left ventricle is dependent on the anterior descending branch; thus, occlusion of the left circumflex does not affect this portion of the left ventricle. Infarction of the lateral (free) wall alone will result from occlusion of the circumflex in a right dominant circulation. An isolated infarct of the posterior interventricular septum arises from occlusion of the posterior descending branch. Isolated infarcts of the right ventricular wall are very rare and would be caused by occlusion of branches of the right coronary artery.

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This is a case of rheumatic fever, which is an immunologically mediated sequela to Streptococcus pyogenes pharyngitis. It is a type II cytotoxic hypersensitivity, involving antibodies that bind to cardiac tissue, activate complement, and thereby cause cell destruction. It is therefore most similar to idiopathic thrombocytopenic purpura, which is also a form of type II cytotoxic hypersensitivity, in this case mediated by antibodies against platelets producing complement fixation and causing the clotting dyscrasia. Atopic allergy is a form of type I hypersensitivity, mediated by IgE antibodies and basophils and mast cells. Contact dermatitis is a form of type IV hypersensitivity mediated by T cells and macrophages. Graft-vs-host disease is a form of type IV hypersensitivity mediated by T cells and macrophages. Graves disease is a form of type II hypersensitivity, but it is NOT cytotoxic in its action. Instead, antibodies to the TSH receptors on thyroid cells cause overstimulation of the gland and its eventual exhaustion. Myasthenia gravis is a form of type II hypersensitivity, but NOT of the cytotoxic variety. In this case, antibodies to the acetylcholine receptors on neurons diminish neurotransmission. Rheumatoid arthritis is a form of type III hypersensitivity, caused by immune complex deposition in joints and subsequent activation of complement. Serum sickness is a form of type III hypersensitivity, caused by immune complex deposition. Systemic lupus erythematosus is a form of type III hypersensitivity, caused by immune complex deposition.

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Systemic thromboembolism may develop in each of these patients. Vegetations associated with infective endocarditis may undergo fragmentation and result in systemic thromboembolism. Stasis develops in dilated ventricles, which predisposes to formation of thrombi attached to the ventricular walls (mural thrombi). Mural thrombi may also form within the left atrium in the presence of mitral valve prolapse. Thromboemboli may originate from mural thrombi. Decreased compliance is a pathophysiologic alteration present in a variety of cardiac disorders in which there is impediment to expansion or relaxation of ventricular walls, such as restrictive cardiomyopathy, hypertrophic cardiomyopathy, and constrictive pericarditis. This feature is not present in any of the conditions described in the question. Depressed myocardial contractility results from conditions that impair myocardial inotropism, such as dilated cardiomyopathy and ischemic heart disease. Depressed inotropism is not present in infective endocarditis or mitral valve prolapse. Of the three conditions in the question stem, only infective endocarditis is definitely related to an infectious etiology, usually bacteria. Recall that mitral valve prolapse is due to myxomatous degeneration of the mitral valve, sometimes associated with Marfan syndrome. The etiology of dilated cardiomyopathy is heterogeneous, and most cases are idiopathic. Of the remaining cases, viral infections, toxic insults (especially alcohol), metabolic disorders (hemochromatosis), pregnancy, and genetic influences are the underlying causes. Mitral valve stenosis may develop as a result of vegetations forming on the mitral valve and occluding the valvular orifice. Endocarditis of the mitral valve more often leads to mitral insufficiency because of destruction of valve leaflets or rupture of chordae tendineae. On the contrary, both mitral valve prolapse (usually clinically silent) and dilated cardiomyopathy may lead to mitral valve insufficiency and regurgitation.

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This patient has disseminated gonococcemia. Gonococcal arthritis and tenosynovitis typically involve both the upper and lower extremities equally. Vesicular skin lesions are characteristic of disseminated gonococcal disease. Females are at particular risk of gonococcemia during menstruation, since sloughing of the endometrium allows access to the blood supply, necrotic tissue enhances the growth of Neisseria gonorrhoeae, and there is an alteration of the pH. Patients who have a C6-8 deficiency have an increased risk of disseminated gonococcemia and tend to have multiple episodes. These patients are also at risk for bacteremia from Neisseria meningitidis. C1 esterase inhibitor deficiency can occur as an autosomal dominant disorder or is acquired. Patients have angioedema without urticaria. The syndrome is also associated with recurrent attacks of colic and episodes of laryngeal edema. Ciliary dysfunction is a marker of Kartagener syndrome (immotile cilia syndrome). The syndrome includes infertility, bronchiectasis, sinusitis, and situs inversus. It is an autosomal recessive disorder caused by abnormalities in the dynein arm of the cilia. Endothelial adhesion molecule deficiency, or beta 2 integrin deficiency, is characterized by failure of neutrophils to express CD18 integrins on their surface. Patients have impaired phagocyte adherence, aggregation, chemotaxis, and phagocytosis of C3b-coated particles. Clinically, there is delayed separation of the umbilical cord, sustained agranulocytosis, recurrent infections of skin and mucosa, gingivitis, and periodontal disease. Eosinophil deficiency, or eosinopenia, occurs with stressors such as acute bacterial infection and following administration of glucocorticoids. There is no known adverse effect of eosinopenia.

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The patient has myocarditis due to Trypanosoma cruzi. This infectious condition, known as Chagas disease, is endemic in vast areas of South America and is transmitted from person to person by triatomids known as “kissing bugs.” Experts assess the number of persons with Chagas disease at about 7 million, with about 35 million at risk in South America. T. cruzi is an intracellular protozoon that localizes mainly in the heart and nerve cells of the myenteric plexus, leading to myocarditis and dysmotility of hollow organs, such the esophagus, colon, and ureter. Cardiac involvement manifests with ventricular dilatation and congestive heart failure secondary to myocyte necrosis and fibrosis. Intracellular parasites can be visualized in tissue sections. Chagas disease is a cause of acquired achalasia, in which the distal third of the esophagus dilates because of loss of its intrinsic innervation. A similar pathologic mechanism accounts for megacolon and megaureter in Chagas disease.

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The rule of thumb is that the plasma concentration will reach 50% in one half-life, 75% in two half-lives, 87.5% in three half-lives, etc., so that the difference between the current drug level and 100% halves with each half-life. In this instance, it takes two half-lives to reach 75%. The half-live of this drug is 6 hours, so two half-lives is 12 hours.

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