Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 471:

  Which of the following addictive drug is sometimes injected for relief from a severe pain?

  A. Heroin

  B. Alcohol

  C. Aspirin

  D. Nicotine

  Correct Answer: A

• Question 472:

  The endosymbiosis theory proposes that mitochondria originated through the engulfing of aerobic prokaryotes by anaerobic host prokaryotes. Many of the enzymes required for aerobic respiration are located on the inner membrane of the

  mitochondria. Where would these enzymes, or their precursors, most likely have been found three billion years ago (prior to endosymbiosis)?

  A. In the cytoplasm of the anaerobic prokaryotic host cells

  B. On the plasma membrane of the anaerobic prokaryotic host cells

  C. On the plasma membrane of the aerobic prokaryotic engulfed cells

  D. In the cytoplasm of the prokaryotic aerobic cells

  Correct Answer: C

  Explanation: As the host cell engulfs a prokaryotic aerobic cell, the host cell membrane forms a vesicle about the aerobic cell, giving a double membrane structure. The inner membrane is formed bythe plasma membrane of the aerobic cell.

  Choice A is incorrect because enzymes in the cytoplasm of the anaerobic host cell would remain in the cytoplasm.

  Choice B is incorrect because enzymes on the plasma membrane of the anaerobic host cell would befound on the outer membrane of the mitochondria formed by endosymbiosis. Choice D is incorrect because enzymes in the cytoplasm of

  the aerobic cell would be within the matrix of the mitochondria formed by endosymbiosis.

• Question 473:

  A mountain climber living at sea level ascends to a very high altitude during the course of a day long climb. By the end of the day, all of the following acclimatizations will occur EXCEPT:

  A. increased tidal volume.

  B. increased respiration rate.

  C. right shift of the hemoglobin dissociation curve.

  D. increased concentration of erythropoietin in the blood.

  Correct Answer: C

  The affinity of hemoglobin for oxygen will not decrease. The adaptation of hemoglobin's affinity for oxygen would take longer than one day. Besides, even if given time to adjust, in order to maintain delivery of oxygen to tissues at high altitudes (low pO2), the affinity of hemoglobin for oxygen would increase so as to maximize the amount of oxygen transported by the hemoglobin in the blood. Choices A, B, and D are incorrect because these are all acclimatizations that occur immediately. The body will make rapid adjustments to the low pO2 of high altitude in order to maintain delivery of oxygen to the tissues. Tidal volume and respiration rate will increase, in order to increase the minute volume (amount of air inhaled and exhaled in a minute). Inhaling more air than normal will compensate for the fact that the inhaled air has less oxygen than normal. In addition, the body will secrete erythropoietin to increase the number of erythrocytes in the blood and thus increase the oxygen carrying capacity of the blood.

• Question 474:

  At its isoelectric point, an amino acid in an electric field will migrate towards the:

  A. anode.

  B. cathode.

  C. basic region.

  D. it will not migrate.

  Correct Answer: D

  At its isoelectric point, an amino acid exists as a zwitterion with positive and negative charges that cancel each other out. Because it has no net charge, it will not migrate in an electric field.

• Question 475:

  Which of the following species will form the most stable radical?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  The stability of free radicals decreases from tertiary>secondary>primary>methyl. Choice A is the only choice that can form a tertiary radical. Choices B, C, and D will form secondary radicals.

• Question 476:

  A hydropathy plot indicates the hydrophilicity (negative values) and hydrophobicity (positive values) of different regions of a protein. According to the hydropathy plot below, how many cellular membrane spanning regions are possible in this protein?

  

  A. 1

  B. 2

  C. 3

  D. 4

  Correct Answer: B

  The hydropathy plot indicates two regions of hydrophobicity. Because the cell membrane has a hydrophobic internal structure, hydrophobic regions of proteins can reside within the membrane.

• Question 477:

  Before birth, the rodent brain is sexually undifferentiated. It is only in the first few days following birth, during a period referred to as the critical period, that the rodent brain differentiates along male or female lines. The hormone testosterone plays a critical role in this development. Specifically, sexual differentiation is determined by the presence of estradiol, an estrogen derivative of testosterone, in certain areas of the brain. Testosterone is converted to estradiol in critical brain cells that contain the enzyme aromatase. To study the effects of testosterone on the neonatal rodent brain, the following experiments were conducted: The above research, combined with additional studies, concluded that testosterone has two "organizational" effects on the male rodent brain: Defeminization Moderate levels of testosterone-derived estradiol during the critical period are sufficient for defeminization of the brain. Defeminization of the rodent brain results in loss of estrogen positive feedback on LH and FSH secretion and the ensuing loss of cyclicity, as well as loss of female sex behavior. Masculinization High levels of estradiol due to high levels of testosterone during the critical period results in masculinization of the brain. Masculinization leads to the induction of male sex behavior including antagonism towards other males and the mounting of females.

  

  Steroid hormones bind to receptors in the nucleus and directly regulate:

  A. the transcription of mRNA.

  B. the translation of protein.

  C. the production of cAMP.

  D. the replication of DNA.

  Correct Answer: A

  Steroid hormones diffuse into the nucleus and bind to receptors. The steroid/receptor complex binds to DNA and regulates mRNA transcription. In contrast, peptide hormones bind to receptors on the surface of the cell and exert their effects through secondary messenger systems. Choice B is incorrect because protein translation by ribosomes is not regulated by steroid hormones. Choice C is incorrect because cAMP production is generally associated with protein hormones binding to cell membrane receptors. It is part of a single transduction mechanism, acting as a second messenger. Choice D is incorrect because DNA replication is not regulated by steroid hormones.

• Question 478:

  Before birth, the rodent brain is sexually undifferentiated. It is only in the first few days following birth, during a period referred to as the critical period, that the rodent brain differentiates along male or female lines. The hormone testosterone plays a critical role in this development. Specifically, sexual differentiation is determined by the presence of estradiol, an estrogen derivative of testosterone, in certain areas of the brain. Testosterone is converted to estradiol in critical brain cells that contain the enzyme aromatase. To study the effects of testosterone on the neonatal rodent brain, the following experiments were conducted:

  

  The above research, combined with additional studies, concluded that testosterone has two "organizational" effects on the male rodent brain: Defeminization Moderate levels of testosterone-derived estradiol during the critical period are sufficient for defeminization of the brain. Defeminization of the rodent brain results in loss of estrogen positive feedback on LH and FSH secretion and the ensuing loss of cyclicity, as well as loss of female sex behavior. Masculinization High levels of estradiol due to high levels of testosterone during the critical period results in masculinization of the brain. Masculinization leads to the induction of male sex behavior including antagonism towards other males and the mounting of females.

  A normal female rat injected with estradiol will respond by:

  A. decreasing secretion of LH from the anterior pituitary.

  B. increasing secretion of LH from the anterior pituitary.

  C. decreasing secretion of GnRH from the hypothalamus.

  D. increasing secretion of progesterone from the follicle.

  Correct Answer: B

  Estradiol has a positive feedback effect on the pituitary, causing it to secrete LH and FSH. Estradiol also has a positive feedback effect on the hypothalamus, causing it to secrete GnRH. Estradiol injection will lead to an LH surge in rodents

  capable of cycling.

  Choice A is incorrect because LH secretion increases.

  Choice C is incorrect because GnRH secretion increases.

  Choice D is incorrect because progesterone is not secreted by the follicle. It is secreted by the corpus luteum or placenta.

• Question 479:

  Maternal lineage can be traced by sequencing the mitochondrial DNA because the mitochondrial genome is derived primarily from the mother. The best explanation for this phenomenon is that:

  A. sperm have no mitochondria and thus cannot contribute to the mitochondrial genome of the offspring.

  B. the zygote divides rapidly, diffusing the paternal mitochondria amongst many cells.

  C. following penetration, the sperm derived mitochondria disintegrate within the egg.

  D. all the genes coding for mitochondria are located on the X chromosome.

  Correct Answer: C

  Upon penetration of the egg, the sperm mitochondria fall apart and are not replicated when divisions begin. This question can best be answered by eliminating the wrong answer choices. Choice A is incorrect because sperm have large numbers of mitochondria used to drive locomotion. Choice B is incorrect because mitochondria are replicated prior to cell division. Also, this choice would not explain the presence of the maternal genome which should also be diffused by division. Choice D is incorrect because many of the genes coding for mitochondrial proteins are located in the mitochondrial DNA.

• Question 480:

  Before birth, the rodent brain is sexually undifferentiated. It is only in the first few days following birth, during a period referred to as the critical period, that the rodent brain differentiates along male or female lines. The hormone testosterone plays a critical role in this development. Specifically, sexual differentiation is determined by the presence of estradiol, an estrogen derivative of testosterone, in certain areas of the brain. Testosterone is converted to estradiol in critical brain cells that contain the enzyme aromatase. To study the effects of testosterone on the neonatal rodent brain, the following experiments were conducted:

  

  The above research, combined with additional studies, concluded that testosterone has two "organizational" effects on the male rodent brain: Defeminization Moderate levels of testosterone-derived estradiol during the critical period are sufficient for defeminization of the brain. Defeminization of the rodent brain results in loss of estrogen positive feedback on LH and FSH secretion and the ensuing loss of cyclicity, as well as loss of female sex behavior. Masculinization High levels of estradiol due to high levels of testosterone during the critical period results in masculinization of the brain. Masculinization leads to the induction of male sex behavior including antagonism towards other males and the mounting of females.

  An adult male rat that is acyclic is observed to mount females. However, the rat also allows itself to be mounted by male rats. According to the passage, the most likely explanation for these observations is that:

  A. the rat is hermaphroditic, with both male and female sex organs.

  B. the rat has been demasculinized.

  C. the rat has been masculinized but not fully defeminized.

  D. the rat has been defeminized but not fully masculinized.

  Correct Answer: C

  The rat has been masculinized and so exhibits male sex behavior. However, the rat has not been fully defeminized in that it exhibits some female sex behavior. Choice A is incorrect because the passage does not discuss hermaphrodites. It focuses on the sexual differentiation of the brain, not the sex organs. Choice B is incorrect because the passage does not discuss demasculinization. Choice D is incorrect. The passage defines masculinization as the development of male sex behavior, which the rat exhibits by mounting females. Thus, the rat has been masculinized. The passage's definition of defeminization includes the loss of female sex behavior. Since the rat still demonstrates some aspects of female sex behavior, (i.e., allowing itself to be mounted by males), it has not been fully defeminized.