Answer Explanation: (C)
Osmosis, also known as simple diffusion when referring to anything other than water, does not
require energy and exploits the chemical component of potential energy to move material from a region of higher concentration
to that of lower concentration. Passive transport moves material from one side of the membrane to the other according to gradient,
but requires a special channel protein and deals with substances that cannot get through the membrane normally. Active transport
always requires energy and moves substances against their concentration. Endocytosis is moving material from the outside of the cell
into the inside, and involves invagination of the membrane around a material that is then brought into the cell as a vesicle. This usually
is accompanied by a receptor that can start the process.
Answer Explanation: (A)
The restriction point is the point of no return in the cell cycle process (the phases of the cell cycle in order,
are G1/G0, R, S, G2, M) and also the last point before the genetic material starts to double. Although the cell is currently making
enough protein to prepare for division, genetic replication has not started yet. Chromatids are identical copies of chromosomes.
In the S, G2, and M phases, there are 92 chromatids in a somatic cell.
Answer Explanation: (B) This answer would best be explained by describing each of the steps of mitosis with its components:
Prophase: dissolution of nucleoli, chromosomes become visible and nuclear envelope disappears, spindle fibers appear
Metaphase: spindle fibers attach to the chromosomes and the metaphase (equatorial) plate appears
Anaphase: chromosomes start to be pulled by their respective poles to opposite ends of the cell
Telophase: known as the early stage of cytokinesis, this is where the spindles will disappear and a nuclear membrane will be apparent
around each set of chromosomes, chromosomes will start to disappear as they become uncoiled
Cytokinesis: formation of two distinct cells
Answer Explanation: (A)
This is a unique and interesting concept specifically in one occurrence in the medical field: Rigor mortis.
The stiffening of muscles after death is caused by an exhaustion of the body’s ATP supply. ATP in the relaxation stage binds to myosin,
which is what releases the myosin head from the sarcomere and allows muscle to relax. When this does not occur, the muscle remains
stiff. Hydrolyzation of the ATP molecule occurs during resting phase and the muscle is once again ready to contract.
Answer Explanation: (B)
This question tests the knowledge of zymogens (enzymes that are secreted in inactive form) and the method
in which pancreatic enzymes are turned on. Lipase would have a significant effect on absorption if it was lost, as it is secreted in active
form. However, the product of trypsinogen, which is trypsin, activates four of the major enzymes of the pancreas (trypsinogen,
chymotrypsinogen, elastinogen, carboxypeptidase) and allow them to break up their respective organic materials into absorbable
components. Thus, loss of this enzyme would have the most extensive effects.
Answer Explanation: (C)
The wording of this answer will be somewhat different in presentation of information normally, so it will
test the understanding of the concept. The main function that surfactant performs is that is decreases surface tension on the inner
lining of the lung. The alveolar surfaces of the lung naturally have an elastic coil pressure that aid in expulsion of gases during the
exhalation phase. However, without surfactant, there would be too much surface tension for them to remain open at the neutral
state, and therefore would collapse and be unavailable for the next breath. Surfactant allows these alveoli to remain open and
available for the next delivery of oxygen.
Answer Explanation: (B)
This question attempts to lead the reader into retrieving memory for the electron transport chain
which is really not necessary. Understanding the definitions of these terms is sufficient, and knowing that the gradient of
H+ goes from high to low from outside to inside through a membrane protein fits the definition of passive transport.
Answer Explanation: (C)
This question aims to assess the knowledge of cardiovascular anatomy. Deoxygenated blood runs from
the inferior and superior vena cavae and into the right atrium. It flows from there to the right ventricle and then up through the
pulmonary vascular system. A hole in the ventricular septum would allow for this blood to be mixed between ventricles. Since the
left ventricle is at a higher pressure, oxygenated blood will flow from the left ventricle to the right.
Answer Explanation: (A)
This question assesses knowledge of the pathway that gives rise to cell types in blood-forming cells.
Platelets, which arise from megakaryocytes and megakaryocytic precursors, arise from the myeloid population. The other
answers that would have been correct are neutrophils, basophils, eosinophils, and macrophages. Lymphoid progenitor cells
are the lymphoid tissue equivalent to myeloid progenitor cells, and give rise to plasma cells, T cells, and NK cells.
Answer Explanation: (B)
Passive immunity is defined as transfer of antibodies from an individual to a patient. This immunity
lasts only for as long as the lifespan of the antibodies (3-6 months) and the patient does not continue to produce them.
Conversely, active immunity is stimulation of the immune system to produce its own antibodies, such as a vaccine. Natural
and artificial are terms to describe how the patient was infected.
Answer Explanation: (A)
This question is important to understand the function of the kidney, and relies on the reader’s knowledge
of what permeability is. Since the descending loop of Henle is only freely permeable to water and no other substances, the concentration
of solutes inside the nephron becomes very high and the water surrounding it very dilute. This is what allows the ascending loop to reabsorb
so much water using the gradient and necessary ions for the body flow with it (eg. Magnesium, chloride, sodium).
Explanation:
Cortisol acts as an inhibitor after its release on the anterior pituitary and the hypothalamus, indicating a sufficient stress
response to the body and blocking release of more CRF. This is very important, as prolonged cortisol can start to harm the
body. Although cortisol does cause vasoconstriction, it is not the feedback mechanism. The body also does not run out of
CRF. CRF-inhibitory factor does not exist.
Answer Explanation: (B)
This question is helpful to understand both menstruation and fertilization. The corpus luteum, after
follicular rupture, lasts for two weeks, and secretes progesterone, which is responsible for preparing the lining of the uterus
for implantation. It is the loss of this hormone that leads the the menstrual phase of the cycle. If a fertilized egg, or blastocyst,
is implanted into the endometrium, however, it secretes B-hCG, which maintains the corpus luteum and allows for survival and
proliferation of the endometrium to begin the pregnancy. This hormone is detectable 1-2 weeks after conception.
Answer explanation: (D)
Steroid hormones, derived from cholesterol, act on intracellular receptors and produce mRNA specific to
the outcome they produce. Thus, they are found both inside and out of the cell. Peptide hormones, such as insulin, act on outer
membrane receptors, but are polarized, so they cannot enter the cell. Amino acid hormones can have the ability to enter the cell,
but are usually made up of very small combinations of amino acids, and never of cholesterol, a four hydroxycarbon ring.
Answer Explanation: (D)
This question requires secondary knowledge to determine not only which refractory period the neurons are at during the second shock,
but to also extrapolate that to a depolarization curve. During depolarization and repolarization, the neuron is in what is called the absolute
refractory period, meaning no amount of stimulus will lead to a firing. During hyperpolarization, if a stronger than normal stimulus is applied,
there will be a response. This is known as the relative refractory period. Resting potential will show a response to the same amount of stimulus
as any other time. This is depicted in the graph above:
Answer Explanation: (A)
Although a pedigree will almost always be a visual question, it is very important to be able to understand
the logic of heritability to increase accuracy on these questions. Because a male can only pass an X chromosome to a daughter,
if he is affected, he will produce all affected daughters. Mitochondrial DNA is passed exclusively by the mother and therefore will
affect all her descendants. Females are twice as likely to be affected as males in X-linked dominant disorders, since they have two X
chromosomes. Finally, X-linked recessive disorders tend more towards males, since they only need one inherited copy to be affected.
Answer Explanation: (C)
The most important concept that needs to be known for enzymes is that they do not change the overall
free energy of a reaction. Rather, they are facilitators of reactions by making the transitional state (The peak of the red curve) easier
to attain, which reduces the activation energy needed. They do not increase either the reactant or the product, and the overall result
of the reaction is the same.
Answer Explanation: (D)
The key of understanding the Michaelis-Menten equation is to view it in terms of the slope of the velocity
increase in relation to the amount of increase of substrate. The slope is the highest below the Km point, which is when half of the
sites of the enzyme are full. After this point, you begin to get diminishing returns on the increase in velocity as substrate is added.
After Vmax, no amount of substrate will increase the rate of reaction.
Answer Explanation: (B)
A vital component to understanding the glycolysis pathway, and one that is not well elicited on the graph
but must be known, is that when fructose 1,6-DP splits in step 4, it produces 2 molecules of glyceraldehyde-3-phosphate. Hence,
every step after this occurs in double. Therefore, although it looks as though 1 molecule of glucose yields 1 molecule of pyruvate,
it is actually the case that all yields after step 4 are doubled. This is an easy way to be tripped up on the exam.
Answer Explanation: (D)
The shifts in the hemoglobin-dissociation curve are important to know to understand how oxygen
gets offloaded into tissues. The essential idea is that areas that need oxygen more than the blood should create environments
that allow oxygen to come off of hemoglobin and into these tissues. The Bohr Effect explains why hemoglobin loses affinity for
oxygen in areas of increased CO2 and decreased pH (these are usually seen together, such as in metabolically active tissue during
exercise). It is important to remember that increased H+ ions and decreased pH are interchangeable. In areas where CO2 decreases
(such as the lungs), hemoglobin in the blood binds to oxygen more readily. BPG stabilizes the low oxygen affinity state of hemoglobin.
The correct answer is: D
When a patient with an asthma attack does not respond to treatment and has been hyperventilating for over an hour, he or she
may become fatigued and may not be able to maintain hyperventilation. In this case, the patient begins to decrease his or her breathing
rate and is not receiving adequate oxygen. By extension, carbon dioxide is trapped in the blood, and the pH begins to drop. Despite the
fact that this pH is normal at the moment, this patient is crashing and may start demonstrating acidemia in the near future. While the kidneys
should compensate for alkalemia, this is a slow process and would not normalize the blood pH within an hour; further, adequate compensation
by the kidneys would actually be a reassuring sign, eliminating choice (B). There is no evidence to believe the measurement was inaccurate,
eliminating choice (A). Finally, after treatment, the patient should return to a normal blood pH with adequate ventilation and would not be expected
to overcompensate by becoming acidemic, eliminating choice (C).
B: Most enzymes in the human body operate at maximal activity around a temperature of 37°C and a pH of 7.4, which is the pH of most body fluids. In addition, as characterized by the Michaelis–Menten equation, enzymes form an enzyme–substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form a product. So far, we can eliminate choices (A), (C), and (D), so let’s check choice (B). An increase in the substrate concentration, while maintaining a constant enzyme concentration, leads to a proportional increase in the rate of the reaction only initially. However, once most of the active sites are occupied, the reaction rate levels off, regardless of further increases in substrate concentration. At high concentrations of substrate, the reaction rate approaches its maximal velocity and is no longer changed by further increases in substrate concentration.
D: Vitamin A is metabolized to retinal, which is important for sight. Vitamin D is metabolized to calcitriol, which is important for calcium regulation. Vitamin E is made up of tocopherols, which are biological antioxidants. Vitamin K is necessary for the introduction of calcium binding sites, such as during the posttranslational modification of prothrombin.
B: The movement of any solute or water by diffusion or osmosis is dependent only on the concentration gradient of that molecule and on membrane permeability.
CORRECT ANSWER D
Explanation:
All of the other statements are true, but the DNA and RNA molecules do not have the same structure.
