Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 401:

  The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.

  During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.

  During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.

  Figure 1: Electronic and pressure changes in the heart and aorta during the cardiac cycle.

  

  The first heart sound represented in Fig. 1 is probably made when:

  A. During ventricular systole, blood in the ventricle is forced against the closed atrioventricular valve.

  B. During ventricular diastole, blood in the ventricles is forced through the aortic and pulmonary artery valves.

  C. During ventricular diastole, blood in the ventricle is forced against the closed atrioventricular valve.

  D. During ventricular systole, blood in the arteries is forced through the aortic and pulmonary artery valves.

  Correct Answer: A

  This entire passage relies little on your previous knowledge or understanding of cardiovascular physiology; rather, emphasis is on your ability to read a graph and, at times, correlate the information with the passage. This question indicates that we should be looking at the graph representing "Heart sounds" in Figure 1. The graph for heart sounds can be described thus: a horizontal line is followed by a narrow burst of spikes or activity; prior to the spikes is the label "1st" implying that the spikes represent the 1st heart sound. The first heart sound begins immediately after the first long vertical line. By following this vertical line downwards, we can note any other events which may initiate or occur at the time as the first heart sound: beginning near the bottom of Figure 1, note that the first heart sound occurs (i) in phase 2 of the cardiac cycle; (ii) at the beginning of ventricular SYSTOLE; (iii) during a period that the aortic and pulmonary valves are not opened

  (i.e. closed); and (iv) the first heart sound begins exactly when the position of the AV valves just become closed. Therefore, strictly according to the figure, during ventricular systole the AV valves suddenly close just as the first heart sound is created. Thus, answer choice A is the only plausible answer.

• Question 402:

  The body compensates for increased environmental temperature by:

  A. decreasing salt retention.

  B. increasing respiration rate.

  C. increasing heart rate.

  D. increasing water lost through skin.

  Correct Answer: D

  The body compensates for increased environmental temperature by sweating and losing water through the skin. Respiration and heart rate increase during exercise in response to internal demand for more oxygen. Salt retention has little to do with environmental temperature.

• Question 403:

  Which of the following phases are common to cells undergoing meiosis and mitosis?

  I. G0 phase

  II. G2 phase

  III.

  S phase

  A.

  I only

  B.

  I and II only

  C.

  II and III only

  D.

  I, II, and III

  Correct Answer: C

  This question asks the examinee to determine a phase within the cell life cycle that is common to both mitotic-dividing cells and meiotic-dividing cells. The cell cycle is composed of interphase and M phase. In mitotic-dividing cells, interphase includes G1 phase, S phase, G2 phase, and in highly specialized cells, such as neurons, a G0 phase. Similarly, meiotic-dividing cells include interphase and M phase, but germ cells do not exist in growth arrest phase (G0). Thus, II and III are correct making answer choice C the best answer.

• Question 404:

  Which structure shows two amino acids linked by 1 peptide bond?

  

  A. option A

  B. option B

  C. option C

  D. option D

  Correct Answer: D

  Explanation: Amino acids are joined by a peptide bond: 1) the leading amino acid has a primary amine group to the left, 2) the peptide bond has a carbonyl group immediately following the first amino acid side chain, 3) bonded to a secondary amine leading the second amino acid side chain, and 4) the second amino acid finishes with a carboxylic acid group. Choice D is correct.

• Question 405:

  A segment of DNA from a lab mouse is determined to be 5’ – GGATCCTCATG – 3’.

  Which of the following DNA segments would be the result of this original DNA sequence experiencing both a point mutation and a deletion?

  A. 5’ – GCATCCTCATG – 3’

  B. 5’ – TGATCCCAG – 3’

  C. 5’ – GGTCCTCATC – 3’

  D. 5’ – GGATCCATG – 3’

  Correct Answer: C

  Explanation: A point mutation is one in which a single DNA base has be replaced by another and a deletion in one in which a single DNA base has been deleted. The question asks us to look for an answer choice that has had both a point mutation and a deletion. Only choice (C) matches these requirements. A, D: Neither (A) nor (D) has had any deletions at all -the length of the DNA chain is unchanged. B: Two deletions have occurred as well as a point mutation. Both the 7th and 10th nucleotide are missing (both T).

• Question 406:

  The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.

  During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.

  During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.

  Figure 1: Electronic and pressure changes in the heart and aorta during the cardiac cycle.

  

  Position P on the EKG of Fig. 1 probably correspond to:

  A. atrial contraction.

  B. ventricular contraction.

  C. the beginning of ventricular systole.

  D. the beginning of ventricular diastole.

  Correct Answer: A

  This question requires you to incorporate information from the figure with information given in the passage. Following vertically down from position P in Figure 1, note that it occurs during a period of diastole. In paragraph 3, sentence 4, the passage gives information concerning the actions of the heart during diastole which would lead to answer choice A. Alternatively, the answer can be deduced through information in the passage and figure. In paragraph 1, sentence 2, diastole is described as a period of ventricular relaxation, ruling out answer choice B. Position P in Figure 1 occurs during a period of diastole and it occurs at a point about halfway through that period, ruling out answer choices C and D.

• Question 407:

  Which of the following molecules will stop being produced first when oxygen is no longer supplied to the cell?

  A. Oxaloacetate

  B. Pyruvate

  C. Water

  D. Adenosine triphosphate

  Correct Answer: C

  This question is testing your understanding of the utilization of oxygen in the production of metabolic energy. Under aerobic conditions, oxygen serves as the final electron accepter in the electron transport chain, which yields water and ATP as final products. In the absence of oxygen, the cell switches from aerobic respiration to anaerobic respiration. As a result, the ETC chain will stop functioning and water will no longer be produced. Oxaloacetate (choice A), an intermediate in the citric acid cycle, is controlled by negative feedback from the ETC. Therefore, the production of oxaloacetate will eventually stop, but the production of water will stop first. Thus, C is the best answer. Without ATP production occurring via aerobic respiration, anaerobic respiration will become the source of ATP for the cell. As a result, glycolysis will increase in order to meet the energy requirements of the cell resulting in an increase in pyruvate (choice B), a product of glycolysis.

• Question 408:

  All of the following are true regarding the function of neurons EXCEPT:

  A. Hyperpolarization at the end of an action potential is one mechanism by which neurons limit the rate at which action potentials may fire.

  B. The flow of sodium into the neuron depolarizes the membrane in the first phase of an action potential.

  C. The transmitting neuron secretes neurotransmitters into the synaptic cleft from its dendrites.

  D. An action potential is initiated when the axon hillock reaches the threshold potential.

  Correct Answer: C

  This question is testing your understanding of the function of action potentials and neuronal transmission of signals. Here, (A), (B), and (D) are all true. Since this is an "EXCEPT" question, the right answer is the one false statement: choice (C). A neuron secretes neurotransmitters from its axon, not its dendrites.

• Question 409:

  DNA and RNA are composed of long chains of monomers known as nucleotides. On the guanine molecule shown below, which arrow best indicates the 5' end of the subunit?

  

  A. I

  B. II

  C. III

  D. IV

  Correct Answer: D

  This question is testing your familiarity of nucleotide structures found in DNA and RNA. Nucleotides are monomers, or subunits, that are the building blocks of DNA and RNA molecules. Each nucleotide consists of a phosphate group, a pentose ring, and a nitrogenous base. In the case of the guanine nucleotide shown, the 5' end corresponds to the phosphate group attached to the fifth carbon on the pentose ring, indicated by the arrow labeled IV. This phosphodiester bond represents the 5' of a nucleotide making choice D the best answer. The arrow labeled III corresponds to the third carbon in the pentose ring. The hydroxyl group attached to the third carbon represents the 3' end of a nucleotide. The arrow labeled II represents the first labeled carbon. Lastly, arrow I points to the nitrogenous base of guanine.

• Question 410:

  DNA polymerase creates new DNA by adding complimentary nucleotides to a template strand from the original double-stranded DNA. If a section of the template strand had a ratio of 3:2 of A:T bases, what is the ratio of A:T in the newly synthesized complimentary strand of DNA?

  A. 3:2

  B. 1:1

  C. 2:3

  D. It cannot be determined.

  Correct Answer: C

  Explanation: DNA polymerase matches complimentary bases to each base in the template strand. If the template strand has an A:T ratio of 3:2, then the complimentary strand must have an A:T ratio of 2:3 in order to match the complimentary bases with their correct paired nucleotide. That is, each A in the original strand will be matched with a T and vice versa.

  A: 3:2, incorrect. This would be true if the complimentary strand matched A:A and T:T.

  B: 1:1, incorrect. Complimentary strand bases are paired in the same ratio, but with T in the place of A.

  C: 2:3, correct.

  D: it cannot be determined, incorrect. A:T must be matched with T:A on the complimentary strand.