Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 411:

  An individual is born with a mutation causing her to partially retain a form of fetal hemoglobin into adulthood. Compared to a normal individual, this person would exhibit:

  A. no differences from a normal adult.

  B. significantly reduced oxygen binding in the lungs.

  C. no symptoms, since retention of fetal hemoglobin would be fatal.

  D. increased oxygen binding to hemoglobin in the tissues.

  Correct Answer: D

  Fetal hemoglobin has a higher affinity for oxygen than does adult hemoglobin. Thus, it binds and holds oxygen more tightly. An individual with fetal hemoglobin would show increased oxygen binding to hemoglobin as a result.

  A ?false, because fetal and adult hemoglobin are different.

  B ?false, because fetal hemoglobin binds more to oxygen, not less. C ?false. In fact, reactivating fetal hemoglobin can be used as a treatment for sickle-cell.

• Question 412:

  

  For a particular plant, straight leaves result from a dominant gene (L) and wrinkled leaves result from a recessive gene (l), and tall height results from a dominant gene (T) while short height results from a recessive gene (t). A dihybrid cross is performed with plants heterozygous for both traits. What is the expected ratio of short, straight-leaved plants to tall, straight-leaved plants?

  A. 1:1

  B. 1:2

  C. 1:3

  D. 3:1

  Correct Answer: C

  In a dihybrid cross, the genes for two traits are crossed in a Punnett square. For two heterozygotes crossed in a Punnett square, the expected ratio of recessive/recessive, dominant/recessive, recessive/ dominant, to dominant is 1:3:3:9. In this particular question, the ratio of both recessive/dominant to dominant/dominant is 1:3.

• Question 413:

  The lining of the digestive tract and the respiratory tract develops from which germ layer?

  I. Endoderm

  II. Mesoderm

  III.

  Ectoderm

  A.

  I only

  B.

  II only

  C.

  III only

  D.

  I and II

  Correct Answer: A

  The lining of the digestive tract and the respiratory tract develops from the endoderm. As a rule of thumb for the MCAT, think of the fetus developing from a tube. Skin and nervous systems develop from ectoderm (outside of tube), digestive tract and respiratory tract from endoderm (inside of tube), and cardiovascular, skeletal, and muscle tissues develop from mesoderm (in-between outside and inside of tube). On MCAT questions, tissues will almost always develop from one germ-layer only.

• Question 414:

  Which of the following are steroid-based molecules?

  I. Testosterone

  II. Triglycerides

  III. Progesterone

  IV.

  DNA

  A.

  I only

  B.

  I, II, and III

  C.

  I and III

  D.

  I, III, and IV

  Correct Answer: C

  Common steroid-based molecules commonly seen on the MCAT include testosterone, progesterone, estrogen, cortisol, and aldosterone.

  a.

  I only, incorrect.

  b.

  I, II, and III, incorrect. Triglycerides consist of three fatty acids bound by ester linkages to a glycerol backbone.

  c.

  I and III, correct.

  d.

  I, III, and IV, incorrect. DNA is not a steroid-based molecule; DNA consists of deoxyribose, a phosphodiester linkage, and a nitrogenous base.

• Question 415:

  

  The amino acid histidine has a nitrogen-containing side chain. The pKa for the carboxylic acid group, imidazole side chain, and the primary amine has a value of 1.8, 6.0, and 9.0, respectively. Which of the following structures accurately reflects the predominate form of histidine at a normal physiological pH of 7.4?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  This question requires the examinee to apply the Henderson-Hasselbalch equation to determine the most common structure of the amino acid histidine at a normal physiological pH. pH = pKa + log[A?HA] To answer this question, you must figure out the predominate form of each acidic substituent group. Carboxylic acid group: 7.4 = 1.8 + log[A?HA]. The logarithm of the ratio of the conjugate base to the acid must equal 5.6. That means the concentration of the conjugate base is

  5.6 orders of magnitude greater (105.6) than the concentration of the acid. Thus, the carboxylic acid will be in the conjugate base form. Imidazole side chain: 7.4 = 6.0 + log[A?HA]. The concentration of the conjugate base is 1.4 orders of

  magnitude greater than the concentration of the acid.

  Therefore, the conjugate base will be the predominate form. Primary amine group: 7.4 = 9.0 + log[A?HA]. The log[A?HA] is equal to -1.6, which means the concentration of the protonated form is 1.6 orders of magnitude greater than the

  concentration of the conjugate base. Accordingly, histidine will have its carboxylic acid and imidazole side chain in their conjugate base form while the primary amine will remain as neutral R-NH2. Thus, C is the correct answer.

• Question 416:

  Diisopropylfluorophosphate (DFP) binds to the active site of acetylcholinesterase (ACE) in the synapses of neurons. When DFP binds to ACE, the ACE enzyme is rendered permanently inactive. This makes DFP a potent toxin, with lethal amounts at less than 100 mg. The interaction between DFP and ACE can best be characterized as:

  A. Competitive inhibition

  B. Noncompetitive inhibition

  C. Irreversible inhibition

  D. Partially competitive inhibition

  Correct Answer: C

  A: Competitive inhibitors bind to the active site but bind reversibly: they do not permanently disable the enzyme. By adding very high levels of the normal substrate, the effects of competitive inhibitors can be overcome.

  B: Noncompetitive inhibitors bind to a site other than the active site

  D: Partially competitive inhibitors bind to a site other than the active site and allow the enzyme to still have some catalytic activity (typically reduced, but possible to be higher).

• Question 417:

  A certain molecule acts by cytochrome oxidase A3, the final enzyme in the electron transport chain. Administration of a large dose of this substance to a human would likely:

  A. lead to death due to an inability of the cell to pass electrons to oxygen, thus stopping aerobic respiration and asphyxiating the cells.

  B. lead to death due to an inadequate supply of ADP to accept a phosphate group at the ATP synthase enzyme.

  C. have no effect as cells would switch which macronutrient they metabolize to circumvent the blocked biochemical pathway.

  D. increase the cell's ATP production as negative feedback would cause the cell to up-regulate anaerobic pathways.

  Correct Answer: A

  The electron transport chain works by taking high-energy electrons from molecules like NADH and passing them through a series of enzymes, ultimately ending with oxygen as the final electron acceptor. If this chain is blocked, aerobic respiration would cease, killing the cell. Eukaryotic cells in humans cannot survive using only anaerobic respiration. Thus (A) is the right answer. Several poisons work in the manner described, most notably cyanide.

  B: ADP is produced when cells use up ATP to carry out cell functions. Blocking the electron transport chain would have no effect on ADP's immediate availability.

  C: As discussed above, this poison would kill the person.

  D: As discussed above, this poison would kill the person.

• Question 418:

  Myoclonic epilepsy and ragged-red fiber (MERRF) is an extremely rare disorder that affects neuromuscular systems. MERRF results from a mutation in mitochondrial DNA (mtDNA) that impairs protein synthesis, oxygen consumption, and energy production. When an affected male and a normal female reproduce, which of the following best predicts the expected phenotypic ratios of the offspring?

  A. None of the offspring will be affected.

  B. All males and no females will be affected.

  C. Half of males and half of females will be affected.

  D. One-fourth of the offspring will be affected.

  Correct Answer: A

  This question is asking the examinee to determine offspring phenotypes for a gene that is located on mitochondrial DNA. Since mtDNA follows maternal inheritance, all the offspring receive their mitochondrial DNA from the mother. In this

  case, the MERRF-causing mutation is located on the father's mtDNA. Therefore, the mutations are not passed on to the offspring. Since all of the offspring will receive functioning mtDNA from the mother, none of the offspring will inherit

  mutated genes causing MERRF.

  Thus, answer choice A correct.

• Question 419:

  A person suffering from severe dehydration and starvation would NOT be expected to have elevated plasma concentrations for which of the following hormones?

  A. Antidiuretic hormone (ADH)

  B. Cortisol

  C. Aldosterone

  D. Insulin

  Correct Answer: D

  This question asks you to determine the body's physiological response to the extreme conditions of dehydration and starvation. In such cases, the person would be expected to have low blood pressure and low nutrients (i.e. proteins, carbohydrates, and lipids). As a result, the body will respond by producing hormones that increase blood pressure and mobilize stored nutrients (glycogen, fat, etc.). ADH and aldosterone both increase blood pressure so they would be expected to appear in high plasma concentrations in response to the low blood pressure, eliminating choices A and C. Cortisol would be expected in high plasma to increase carbohydrate, protein, and fat availability, making B false. That leaves answer choice D. Insulin lowers blood glucose levels, which is expected to appear in the blood after a sugary meal. In the case of starvation, the body would respond by increasing blood glucose levels via the production of glucagon.

• Question 420:

  In nerve cells, microtubule-associated proteins (MAPs), most notably MAP2 and MAP tau, act to stabilize microtubules. In a mouse model, a mutant is developed that vastly reduced function across all families of MAPs, leading to increased microtubule degradation. Which cellular activity would likely be most affected?

  A. Cardiac muscle contraction

  B. Transcription of mRNA from DNA

  C. Krebs cycle

  D. Meiosis

  Correct Answer: D

  Microtubules are a component of the cytoskeleton and central to a number of cellular processes that involve movement, such as mitosis and meiosis. They provide the framework for the organization and separation of chromosomes and sister chromatids during that process. Thus (D) is the correct answer.

  A: The movement created by muscle cells depends on actin and myosin, rather than microtubules.

  B: Transcription is primarily regulated by enzymes, not microtubules.

  C: The Krebs cycle depends primarily on mitochondria, not microtubules.