MCAT MCAT Certifications MCAT-TEST Questions & Answers

• Question 31:

  Fluoroscopy is an imaging technique that uses X-rays to obtain real-time moving images of the interior of the body. A patient was asked to perform cycles of deep inspiration and deep expiration. Fluoroscopy was used to measure the linear velocity of the movement of the diaphragm and the data was plotted against time. The origin in Figure 1 is the reference time 0 when the diaphragm was essentially in its equilibrium position.

  

  At what time after t = 0 is the displacement of the diaphragm at a minimum?

  A. A

  B. B

  C. C

  D. D

  Correct Answer: C

  To determine the displacement (NOT distance), take the area under the graph. The 2 similar sized trapezoids, one positive and the other negative, cancel each other. At time C, the diaphragm is back or near to its starting position; hence, its displacement has zero magnitude at that moment.

• Question 32:

  The value of the water dissociation constant Kw varies with temperature. Its value is normally given as 1.00 x 10-14 mol2 dm-6 at room temperature but 1.00 x 10-13 mol2 dm-6 at 60 °C. What is the pH of pure water at 60 °C?

  A. Equal to 7.0 thus the water is neutral.

  B. Less than 7.0 thus the water is acidic.

  C. Less than 7.0 but the water is basic.

  D. Less than 7.0 but the water is neutral.

  Correct Answer: D

  It is not necessary to calculate anything to solve this problem but we will go through the steps.

  The dissociation of water (note carefully that the ratio of the products is 1:1; also keep in mind that this is the basis of the neutrality of pure water: acid units = base units):

  H2O + H2O <-> H3O+ + OH-

  The expression for the self-ionization of water or the water dissociation constant:

  Kw = [H3O+][OH-] = [H+][OH-]

  At room temperature, keeping in mind that the dissociation of the ions is 1:1, we get: 10-14 = [H+][OH-] = [H+]2. And so [H+] = 10-7 and then -log[H+] = 7 = pH.

  Doing the identical math but using Kw = 10-13 yields a pH of 6.5. BUT because the ratio of the ions is still 1:1 (if not, you would not have done the calculation) thus it must be neutral.

• Question 33:

  A spherical air bubble is rising in a test tube which is illuminated from one side by a flashlight. What happens to a ray of light incident on the air bubble at A?

  

  A. Reflection at both surfaces

  B. Reflection at A; no reflection at B

  C. Reflection at B; no reflection at A

  D. no reflection at either surface

  Correct Answer: A

  Reflection is the process by which light rays bounce back into a medium from a surface with another medium. Note that there are 2 surfaces, both A and B, which meet the criterion for reflection. Remember: just because some light reflects, does not mean that all of the light reflects. Specifically, some of the light will transmit completely through the bubble while some would have been reflected at A and some at B.

• Question 34:

  The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?

  A. H3N+-CH2-COO

  B. H2N-CH2-COOH

  C. H3N+-CH2-COOH

  D. H2N-CH2-COO-

  Correct Answer: A

  At pH 6.0, the isoelectric point, glycine is electrically neutral. This corresponds to the zwitterion form shown below. (Note that the form in answer B, though commonly written as short hand, is impossible to find in nature.)

  H3N+-CH2-COO-

  Note the logic of the above zwitterion: the base component (the amino group) has accepted a proton from the acid component (the carboxylic acid COOH group). Answer choice C dominates below the isoelectric point (e.g. in a more protonated environment), and answer choice D dominates above the isoelectric point (e.g. in a less protonated environment).

  Although it is possible to get a question like this on the real exam, it would be considered relatively easy! For the real exam, you will certainly have some questions involving amino acids that are dependent on your memory of the features of the side chains of the 20 common protein-building amino acids, how their charges change with pH, and their 3-letter and 1-letter representations. We will certainly be exploring all of these issues in the exams to follow. Hopefully, this will be a reminder following your MCAT content review:

  

• Question 35:

  Consider the following structure.

  

  Which of the following is the most accurate description of the structure provided?

  A. Dipeptide

  B. Tripeptide

  C. Tetrapeptide

  D. Pentapeptide

  Correct Answer: C

  First a note regarding nomenclature: The shortest peptides are dipeptides, consisting of 2 amino acids joined by a single peptide bond, followed by tripeptides (3 amino acids, 2 peptide bonds), tetrapeptides (4 amino acids, 3 peptide bonds), pentapeptides (5 amino acids, 4 peptide bonds), etc. A peptide (= amide) bond is a covalent bond formed when the carboxyl group of one amino acid reacts with the amino group of another. In other words, focus on the number of times that you see C=O connected to N, as long as that bond is WITHIN the molecule (i.e. between amino acids). Since there are 3 such instances in the molecular structure provided, given the information provided above regarding 3 peptide bonds, the molecule must be a tetrapeptide. Going from left to right in the structure in the image, here are the 4 amino acids: Tyr-Pro-Phe-Pro-NH2

  Going deeper, the rules above apply to linear oligo/polypeptides. However, cyclic oligo/polypeptides will always have 1 additional bond where the amino end and carboxyl end bond to complete the ring.

  Beyond Required Assumed Knowledge (but the following includes some commonly explored ideas in MCAT Biochemistry passages): The tetrapeptide in the image is `morphiceptin'. It is an opioid (= a selective opioid receptor agonist/ligand) so that it has analgesic effects like opium and those effects are reversed by naloxone (= opioid inhibitor; `blocker'; used to block the effects of opioids especially in overdose).

• Question 36:

  Glycogen storage disease type V, also known as GSD-V or McArdle disease, is an autosomal recessive disease that results in the deficiency of myophosphorylase, an isoform of glycogen phosphorylase found in muscle cells. Patients with GSD-V experience severe muscle cramps after strenuous exercise and exercise intolerance.

  Physicians may order two histology stains of the patient's muscle tissue in order to aid in the diagnosis (see Figure 1):

  (A)

  A Periodic acid-Schiff (PAS) stain uses periodic acid to detect carbohydrates in tissues. The reaction of the acid with sugar cleaves vicinal diols creating ketone and/or aldehyde fragments, the latter of which then reacts with the Schiff reagent to give a purple color;

  (B)

  A phosphorylase stain identifies the presence of the enzyme using a dark blue color indicator.

  

  Figure 1A. Comparative histochemistry of GSD-V and healthy individual.

  PAS stain of muscle tissue shows an accumulation of glycogen in the GSD-V individual (top) compared to the control (bottom). B) Phosphorylase stain of muscle tissue reveals an absence of phosphorylase in the GSD-V individual (top).

  Despite initial pain during exercise, many patients with GSD-V have been able to increase their exercise tolerance by engaging in moderate periods of aerobic exercise. Muscle pain and fatigue subsides after a few minutes, a response that

  researchers call the "second wind" phenomenon.

  Patients who experienced "second wind" typically experienced lowered heart rate and a reported decrease in exercise effort after 7-10 minutes. A similar effect was seen in the same patients after an intravenous infusion of glucose.

  

  Figure 2. Measured heart rates in two GSD-V patients during sustained exercise.

  Two subjects were asked to ride stationary bicycles at a steady rate over the course of 40 minutes. The subjects' heart rates were measured continuously, with high and low values coinciding with 7-minute intervals. Glucose was injected

  intravenously after 21 minutes. SW = Second Wind.

  Adapted from Bhavaraju-Sanka R, Howard J. Jr, Chahin N (2014). SOJ Neurol 1(1), 1-3. and Haller RG, Vissing J. Arch Neurol. 2002;59(9):1395-1402.

  For a patient affected by GSD-V, which one of these scenarios describes the concentrations of metabolism products in muscle cells after exercise?

  A. Increased concentrations of ADP, but decreased concentrations of Pi.

  B. Increased concentrations of ATP, but decreased concentrations of Pi.

  C. Decreased concentrations of ATP, and increased concentrations of Pi.

  D. Decreased concentrations of ADP, and increased concentrations of Pi.

  Correct Answer: C

  Because of the deficiency in glycogen phosphorylase, glucose is not released from glycogen, which decreases the production of ATP by glycolysis. Thus, any ATP used in muscles during exercise is not replenished, leading to an accumulation of ADP and Pi.

• Question 37:

  Glycogen storage disease type V, also known as GSD-V or McArdle disease, is an autosomal recessive disease that results in the deficiency of myophosphorylase, an isoform of glycogen phosphorylase found in muscle cells. Patients with GSD-V experience severe muscle cramps after strenuous exercise and exercise intolerance.

  Physicians may order two histology stains of the patient's muscle tissue in order to aid in the diagnosis (see Figure 1):

  (A)

  A Periodic acid-Schiff (PAS) stain uses periodic acid to detect carbohydrates in tissues. The reaction of the acid with sugar cleaves vicinal diols creating ketone and/or aldehyde fragments, the latter of which then reacts with the Schiff reagent to give a purple color;

  (B)

  A phosphorylase stain identifies the presence of the enzyme using a dark blue color indicator.

  

  Figure 1A. Comparative histochemistry of GSD-V and healthy individual.

  PAS stain of muscle tissue shows an accumulation of glycogen in the GSD-V individual (top) compared to the control (bottom). B) Phosphorylase stain of muscle tissue reveals an absence of phosphorylase in the GSD-V individual (top).

  Despite initial pain during exercise, many patients with GSD-V have been able to increase their exercise tolerance by engaging in moderate periods of aerobic exercise. Muscle pain and fatigue subsides after a few minutes, a response that

  researchers call the "second wind" phenomenon.

  Patients who experienced "second wind" typically experienced lowered heart rate and a reported decrease in exercise effort after 7-10 minutes. A similar effect was seen in the same patients after an intravenous infusion of glucose.

  

  Figure 2. Measured heart rates in two GSD-V patients during sustained exercise.

  Two subjects were asked to ride stationary bicycles at a steady rate over the course of 40 minutes. The subjects' heart rates were measured continuously, with high and low values coinciding with 7-minute intervals. Glucose was injected

  intravenously after 21 minutes. SW = Second Wind.

  Adapted from Bhavaraju-Sanka R, Howard J. Jr, Chahin N (2014). SOJ Neurol 1(1), 1-3. and Haller RG, Vissing J. Arch Neurol. 2002;59(9):1395-1402.

  The "second wind" phenomena experienced by GSD-V patients is most likely due to a(n):

  A. activation of phosphofructosekinase-1.

  B. decrease in venous lactic acid.

  C. metabolic switching to oxidative phosphorylation.

  D. increase in insulin.

  Correct Answer: C

  In GSV patients, since glycogen is not broken down into glucose due to a deficiency of myophosphorylase, the body switches to aerobic exercise very quickly. Once anaerobic exercise gives way to aerobic exercise, glycolysis is no longer the primary source of ATP production. Aerobic exercise allows fatty acids to be used as an energy source, generating ATP from oxidative phosphorylation.

• Question 38:

  Glycogen storage disease type V, also known as GSD-V or McArdle disease, is an autosomal recessive disease that results in the deficiency of myophosphorylase, an isoform of glycogen phosphorylase found in muscle cells. Patients with GSD-V experience severe muscle cramps after strenuous exercise and exercise intolerance.

  Physicians may order two histology stains of the patient's muscle tissue in order to aid in the diagnosis (see Figure 1):

  (A)

  A Periodic acid-Schiff (PAS) stain uses periodic acid to detect carbohydrates in tissues. The reaction of the acid with sugar cleaves vicinal diols creating ketone and/or aldehyde fragments, the latter of which then reacts with the Schiff reagent to give a purple color;

  (B)

  A phosphorylase stain identifies the presence of the enzyme using a dark blue color indicator.

  

  Figure 1A. Comparative histochemistry of GSD-V and healthy individual.

  PAS stain of muscle tissue shows an accumulation of glycogen in the GSD-V individual (top) compared to the control (bottom). B) Phosphorylase stain of muscle tissue reveals an absence of phosphorylase in the GSD-V individual (top).

  Despite initial pain during exercise, many patients with GSD-V have been able to increase their exercise tolerance by engaging in moderate periods of aerobic exercise. Muscle pain and fatigue subsides after a few minutes, a response that

  researchers call the "second wind" phenomenon.

  Patients who experienced "second wind" typically experienced lowered heart rate and a reported decrease in exercise effort after 7-10 minutes. A similar effect was seen in the same patients after an intravenous infusion of glucose.

  

  Figure 2. Measured heart rates in two GSD-V patients during sustained exercise.

  Two subjects were asked to ride stationary bicycles at a steady rate over the course of 40 minutes. The subjects' heart rates were measured continuously, with high and low values coinciding with 7-minute intervals. Glucose was injected

  intravenously after 21 minutes. SW = Second Wind.

  Adapted from Bhavaraju-Sanka R, Howard J. Jr, Chahin N (2014). SOJ Neurol 1(1), 1-3. and Haller RG, Vissing J. Arch Neurol. 2002;59(9):1395-1402.

  Muscle biopsies offer valuable data that can aid physicians in the diagnosis. However, they can be painful and invasive. A less invasive test uses a blood pressure cuff placed on the upper arm, which blocks blood flow to the arm. The patient

  affected by GSD-V is then asked to perform a strenuous arm exercise, such as squeezing a rubber ball. After some time, blood is drawn, and its chemical contents are compared to pre-exercise blood samples. One would expect to see:

  A. a decrease in calcium.

  B. an increase in lactate.

  C. an increase in glucose.

  D. no increase in lactate.

  Correct Answer: D

  A deficiency of glycogen phosphorylase in muscles indicates that glucose is not being released from glycogen. Thus, lactic acid fermentation is not occurring, and lactate will not build up in the cells. There will be no increase in lactate levels in the post-exercise blood sample.

• Question 39:

  Glycogen storage disease type V, also known as GSD-V or McArdle disease, is an autosomal recessive disease that results in the deficiency of myophosphorylase, an isoform of glycogen phosphorylase found in muscle cells. Patients with GSD-V experience severe muscle cramps after strenuous exercise and exercise intolerance.

  Physicians may order two histology stains of the patient's muscle tissue in order to aid in the diagnosis (see Figure 1):

  (A) A Periodic acid-Schiff (PAS) stain uses periodic acid to detect carbohydrates in tissues. The reaction of the acid with sugar cleaves vicinal diols creating ketone and/or aldehyde fragments, the latter of which then reacts with the Schiff reagent to give a purple color;

  (B) A phosphorylase stain identifies the presence of the enzyme using a dark blue color indicator.

  

  Figure 1A. Comparative histochemistry of GSD-V and healthy individual.

  PAS stain of muscle tissue shows an accumulation of glycogen in the GSD-V individual (top) compared to the control (bottom). B) Phosphorylase stain of muscle tissue reveals an absence of phosphorylase in the GSD-V individual (top).

  Despite initial pain during exercise, many patients with GSD-V have been able to increase their exercise tolerance by engaging in moderate periods of aerobic exercise. Muscle pain and fatigue subsides after a few minutes, a response that

  researchers call the "second wind" phenomenon.

  Patients who experienced "second wind" typically experienced lowered heart rate and a reported decrease in exercise effort after 7-10 minutes. A similar effect was seen in the same patients after an intravenous infusion of glucose.

  

  Figure 2. Measured heart rates in two GSD-V patients during sustained exercise.

  Two subjects were asked to ride stationary bicycles at a steady rate over the course of 40 minutes. The subjects' heart rates were measured continuously, with high and low values coinciding with 7-minute intervals. Glucose was injected

  intravenously after 21 minutes. SW = Second Wind.

  Adapted from Bhavaraju-Sanka R, Howard J. Jr, Chahin N (2014). SOJ Neurol 1(1), 1-3. and Haller RG, Vissing J. Arch Neurol. 2002;59(9):1395-1402.

  Considering the information provided and your knowledge of glucose and fructose, which of the following is MOST likely to be accurate regarding the reaction with periodic acid?

  A. The reaction would break the cyclic structures of glucose and fructose, with glucose having an additional fragment per molecule.

  B. The reaction would break the cyclic structures of glucose and fructose, each into equal fragments.

  C. The reaction would break the cyclic structure of glucose but not fructose.

  D. The reaction would neither break the cyclic structure of glucose nor fructose.

  Correct Answer: A

  The key points:

  1. Periodic acid will react with carbohydrates. Since glucose and fructose are carbohydrates, we expect that there will be a reaction and the location will be at the vicinal diols (according to P2 of the passage). There is nothing to suggest that the form (either cyclic or linear) is relevant. In fact, for many monosaccharides (including glucose and fructose), the cyclic forms predominate, and it is the cyclic form of glucose that polymerizes to make the chains which form glycogen, and it is glycogen which is the purpose of the PAS test in McArdle disease.

  2. `diol' refers to 2 alcohol/hydroxyl (OH) groups, and a vicinal diol is where the two hydroxyl groups occupy vicinal positions, meaning they are attached to adjacent atoms.

  3. Glucose and fructose are structural isomers (= same molecular formula but different order and/or kinds of bonds). Fructose is a ketone in its linear form while glucose is an aldehyde. Through an intramolecular nucleophilic addition reaction between a carbonyl group and an OH group of the same molecule, glucose forms a six-membered ring (5 carbons + 1 oxygen) while fructose forms a five-membered ring (4 carbons + 1 oxygen). Because there is more than 1 type of atom within the rings, glucose and fructose are 'heterocycles'.

  4. Comparing the 2 structures: glucose has 3 points where there are vicinal diols available for oxidative cleavage by periodic acid while fructose has 2 points.

  Note that glucose (below) has vicinal diols at C1-C2, C2-C3 and C3-C4 and thus cleavage would be expected at those locations resulting in aldehydes or ketones. Note that C4-C5 is not a vicinal diol since there is no 璒H group connected directly to C5.

  

  Fructose (below) has vicinal diols at C2-C3 and C3-C4.

  

  Side note. The blue substituents, in the structure provided for glucose above, are attached to C1, the anomeric carbon. Since OH is pointing up in the Haworth projection, it is the beta anomer, as opposed to the alpha anomer of glucose which differs only in that the OH is pointing downwards.

• Question 40:

  Glycogen storage disease type V, also known as GSD-V or McArdle disease, is an autosomal recessive disease that results in the deficiency of myophosphorylase, an isoform of glycogen phosphorylase found in muscle cells. Patients with GSD-V experience severe muscle cramps after strenuous exercise and exercise intolerance.

  Physicians may order two histology stains of the patient's muscle tissue in order to aid in the diagnosis (see Figure 1):

  (A) A Periodic acid-Schiff (PAS) stain uses periodic acid to detect carbohydrates in tissues. The reaction of the acid with sugar cleaves vicinal diols creating ketone and/or aldehyde fragments, the latter of which then reacts with the Schiff

  reagent to give a purple color;

  (B)

  A phosphorylase stain identifies the presence of the enzyme using a dark blue color indicator.

  

  Figure 1A. Comparative histochemistry of GSD-V and healthy individual.

  PAS stain of muscle tissue shows an accumulation of glycogen in the GSD-V individual (top) compared to the control (bottom). B) Phosphorylase stain of muscle tissue reveals an absence of phosphorylase in the GSD-V individual (top).

  Despite initial pain during exercise, many patients with GSD-V have been able to increase their exercise tolerance by engaging in moderate periods of aerobic exercise. Muscle pain and fatigue subsides after a few minutes, a response that

  researchers call the "second wind" phenomenon.

  Patients who experienced "second wind" typically experienced lowered heart rate and a reported decrease in exercise effort after 7-10 minutes. A similar effect was seen in the same patients after an intravenous infusion of glucose.

  

  Figure 2. Measured heart rates in two GSD-V patients during sustained exercise.

  Two subjects were asked to ride stationary bicycles at a steady rate over the course of 40 minutes. The subjects' heart rates were measured continuously, with high and low values coinciding with 7-minute intervals. Glucose was injected

  intravenously after 21 minutes. SW = Second Wind.

  Adapted from Bhavaraju-Sanka R, Howard J. Jr, Chahin N (2014). SOJ Neurol 1(1), 1-3. and Haller RG, Vissing J. Arch Neurol. 2002;59(9):1395-1402.

  Biopsies of the patients showed glycogen accumulation in muscle cells (Figure 1A), but not in the liver. What is the most likely explanation for this observation?

  A. Glycogen is not stored in the liver.

  B. Liver cells use a different isomer of glycogen phosphorylase.

  C. Glycogen is transported out of the liver via carrier-mediated transport.

  D. Muscles do not require ATP.

  Correct Answer: B

  Myophosphorylase is stated in the passage to be an isoform of glycogen phosphorylase found specifically in muscle cells (also, the name "myophosphorylase" implies that it is a phosphorylase for the muscle ?`myo'). Thus, organs such as the liver, kidneys, or heart would likely not be directly affected by GSD-V. The other answers are incorrect: glycogen is stored in the liver, and muscles require a large amount of ATP, which also explains the symptoms of GSD-V.