MD - Doctor of Medicine Infectious Disease and Microbiology Questions and Answers — Questions and Answers
Question 1: A 45-year-old man with a history of a recent dental procedure followed by a prolonged course of clindamycin presents with profuse, watery, non-bloody diarrhea and lower abdominal cramping. Stool assay is positive for toxins A and B. What is the primary mechanism of action of these toxins produced by the causative organism?
- Activation of adenylate cyclase, leading to increased intracellular cAMP and massive fluid secretion.
- Inactivation of Rho family GTPases, causing cytoskeletal disruption and epithelial cell death. (Correct answer)
- Inhibition of protein synthesis by blocking the EF-2 translocation step.
- Superantigen activity, leading to widespread T-cell activation and cytokine release.
Correct answer: Inactivation of Rho family GTPases, causing cytoskeletal disruption and epithelial cell death.
The patient's presentation is classic for Clostridioides difficile infection (CDI), often precipitated by antibiotics like clindamycin. The primary virulence factors are Toxin A (TcdA) and Toxin B (TcdB), which are glucosyltransferases. They inactivate small GTP-binding proteins of the Rho family within intestinal epithelial cells, leading to actin cytoskeleton disruption, cell rounding, apoptosis, and loss of epithelial barrier function, which results in colitis and diarrhea. [1, 7, 9]
Question 2: A 30-year-old female is diagnosed with her first episode of genital herpes. Which antiviral medication's mechanism of action relies on initial phosphorylation by a viral-encoded thymidine kinase, making it highly specific for infected cells?
- Foscarnet
- Oseltamivir
- Acyclovir (Correct answer)
- Maraviroc
Correct answer: Acyclovir
Acyclovir is a guanosine analog that is selectively effective against herpes simplex virus (HSV) and varicella-zoster virus (VZV). Its selectivity comes from the first step of its activation, which requires phosphorylation by a viral-specific thymidine kinase. [3, 6, 19] Once converted to acyclovir monophosphate, host cell kinases further phosphorylate it to the active triphosphate form, which then inhibits viral DNA polymerase and causes chain termination. [3, 6] Uninfected cells lack this viral enzyme, so the drug remains largely inactive, minimizing toxicity. [19]
Question 3: A 58-year-old male with acute myeloid leukemia is undergoing induction chemotherapy and develops profound neutropenia. He becomes febrile and develops pleuritic chest pain and a cough productive of blood-streaked sputum. A high-resolution CT scan of the chest reveals several nodules, one of which has a surrounding area of ground-glass opacity. This radiologic finding is known as the halo sign. Which of the following is the most likely causative organism?
- Pneumocystis jirovecii
- Candida albicans
- Aspergillus fumigatus (Correct answer)
- Mycobacterium tuberculosis
Correct answer: Aspergillus fumigatus
In a severely immunocompromised, neutropenic patient, the combination of fever, pleuritic chest pain, and hemoptysis with a CT finding of a pulmonary nodule surrounded by a halo of ground-glass attenuation is highly suggestive of invasive pulmonary aspergillosis (IPA). [4, 15, 16] The central nodule represents fungal-induced infarction, and the surrounding halo represents alveolar hemorrhage, characteristic of the angioinvasive nature of Aspergillus. [15]
Question 4: A 65-year-old woman is brought to the emergency department from a nursing home with a suspected urinary tract infection. She is lethargic, her blood pressure is 85/45 mmHg despite a 1-liter crystalloid bolus, and her serum lactate is 5.0 mmol/L. According to the Surviving Sepsis Campaign's 'Hour-1 Bundle', which of the following is a critical and time-sensitive intervention that must be initiated immediately?
- Administering intravenous hydrocortisone
- Obtaining blood cultures and administering broad-spectrum antibiotics (Correct answer)
- Placing a central venous catheter for CVP monitoring
- Initiating continuous renal replacement therapy
Correct answer: Obtaining blood cultures and administering broad-spectrum antibiotics
The Surviving Sepsis Campaign 'Hour-1 Bundle' emphasizes several key interventions to be initiated immediately upon recognition of sepsis or septic shock. Among the highest priorities are obtaining blood cultures before starting antibiotics and then promptly administering broad-spectrum antibiotics, ideally within the first hour. [22, 23, 35] Early and appropriate antibiotic administration is one of the few interventions proven to reduce mortality in septic shock. [5, 8]
Question 5: A 35-year-old patient with a known history of HIV, non-adherent to antiretroviral therapy, presents with a new-onset seizure. His CD4 count is 48 cells/µL. A contrast-enhanced MRI of the brain reveals multiple ring-enhancing lesions, primarily located in the basal ganglia. What is the most likely diagnosis?
- Primary CNS lymphoma
- Progressive Multifocal Leukoencephalopathy (PML)
- Cerebral Toxoplasmosis (Correct answer)
- Cryptococcoma
Correct answer: Cerebral Toxoplasmosis
In an HIV-positive patient with a CD4 count below 100 cells/µL, the presence of multiple ring-enhancing brain lesions, particularly in the basal ganglia, is highly characteristic of cerebral toxoplasmosis, caused by the reactivation of latent Toxoplasma gondii infection. [26, 27, 38] While primary CNS lymphoma is a key differential, it more commonly presents as a single lesion. Given the classic presentation, empiric treatment for toxoplasmosis is typically initiated. [29]
Question 6: A 22-year-old male presents to a sexual health clinic with a two-day history of purulent urethral discharge and dysuria. A Gram stain is performed on a smear of the discharge. Which of the following microscopic findings would be most diagnostic for Neisseria gonorrhoeae infection in this patient?
- Gram-positive cocci in chains
- Gram-negative rods arranged in a 'school of fish' pattern
- Absence of organisms but numerous polymorphonuclear leukocytes
- Gram-negative diplococci located within neutrophils (Correct answer)
Correct answer: Gram-negative diplococci located within neutrophils
The classic and highly specific Gram stain finding for urethritis caused by Neisseria gonorrhoeae in a symptomatic male is the presence of gram-negative (pink/red) diplococci (cocci in pairs, often kidney-bean shaped) inside of polymorphonuclear leukocytes (neutrophils). [11, 21, 33] This finding has a high sensitivity (>95%) and specificity (>99%) in this clinical context. [37]
MD - Doctor of Medicine Infectious Disease and Microbiology Questions and Answers
A 45-year-old man with a history of a recent dental procedure followed by a prolonged course of clindamycin presents with profuse, watery, non-bloody diarrhea and lower abdominal cramping.
Stool assay is positive for toxins A and B.
What is the primary mechanism of action of these toxins produced by the causative organism?