USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 491:

  The process of RNA editing leads to sequence changes that are not reflective of the genetic information contained within the gene for an edited RNA. RNA editing occurs through the action of specific nucleotide deaminases that change either adenine to inosine or cytidine to uridine. These nucleotide changes can affect the coding or splicing capacity of the RNA. Which of the following proteins is an important example of the result of RNA editing?

  A. apo B48

  B. calcitonin gene-related peptide

  C. ferritin

  D. glutamate dehydrogenase

  E. tissue factor (factor III)

  Correct Answer: A

• Question 492:

  Carnitine, a zwitterionic compound derived from lysine, is involved in fatty acid metabolism and is required at which two points in the transport of fatty acids from the cytoplasm to the mitochondria (see below figure)?

  

  A. A and E

  B. A and C

  C. B and C

  D. B and D

  E. E and D

  Correct Answer: D

  Section: Biochemistry Fatty acids are activated to the CoA derivatives at the outer mitochondrial membrane (catalyzed by enzyme 1 in below figure), but are oxidized inside the mitochondria. Long-chain fatty acyl-CoA molecules do not cross the mitochondrial membrane. A special transport system involving carnitine is used for movement of the activated fatty acids into the mitochondria. The fatty acyl moiety is transferred from the CoA to carnitine in a reaction catalyzed by carnitine acyltransferase I (enzyme 2 in below figure). Acyl carnitine thus formed is shuttled across the inner mitochondrial membrane (via enzyme 3 in below figure), where the fatty acyl group is transferred back to a CoA molecule within the mitochondrial matrix. The latter reaction is catalyzed by carnitine acyltransferase II (enzyme 4 in below figure). Medium-chain fatty acyl-CoAs can cross the mitochondrial membrane and do not require the carnitine transport system for entry into the mitochondria. The compound(s) at point Aare ATP and CoA, point B is carnitine, point C is acylcarnitine, point D is carnitine, and E is acylcarnitine. Therefore, the requirements for carnitine occur on either side of the inner mitochondrial membrane at points B and D, which occur during the translocation process. All other combinations (choices A, B, C, and E) have a compound or compounds other than or in addition to carnitine.

• Question 493:

  The term "genomic imprinting" refers to the phenomenon of gene expression dependence on the mode of inheritence. A typical example of this phenomenon is the control of the expression of the growth factor IGF- II. Genomic imprinting, termed "epigenesis," is defined by which of the following?

  A. Gene expression results from regulated levels of DNA methylation.

  B. Gene expression that is restricted to a specific cell lineage.

  C. Gene regulation is exerted by sex-type specifc factors.

  D. Genotype differences are not reflected by phenotype differences.

  E. Phenotype differences are independent of genotype variation.

  Correct Answer: E

  Section: Biochemistry Epigenesis refers to the phenomenon where phenotypic differences are present in the same species, but this difference is not due to differences in the genotype of the organism. None of the other examples (choices AD) define epigenesis.

• Question 494:

  The bacterial lac operon is controlled by both glucose and lactose levels. Which of the following conditions would result in the greatest level of transcription from the lac operon?

  A. both glucose and lactose present

  B. glucose present but no lactose

  C. lactose present but no glucose

  D. no glucose or lactose present

  Correct Answer: C

  Section: Biochemistry The lac operon in bacteria is a group of three coordinately regulated genes whose functions are to allow the bacterium to survive when glucose is low or absent as an energy source. The major nonglucose source of energy is from lactose, a disaccharide of glucose and galactose. The control of expression from the lac operon by glucose is exerted through the level of cAMP, whereas, control by lactose is exerted through induced inhibition of the repressor of the operon. When glucose levels are plentiful, the level of cAMP is low. The role of cAMP in the regulation of the lac operon is to bind to the catabolite activator protein (CAP), which is a DNA-binding factor that stimulates the activity of RNA polymerase. As glucose levels drop, cAMP levels rise with a concomitant increase in CAP-mediated activation of polymerase. In the presence of lactose, the lac operon repressor protein is prevented from binding to the operator region of the operon resulting in an increase in the rate of transcription. Therefore, maximal activity would be seen with no glucose in the presence of lactose. With both glucose and lactose present (choice A), the level of cAMP would be low so there would be no activation of polymerase via CAP, but some level of transcription would occur due to the lactosemediated inhibition of the repressor activity. With glucose and no lactose (choice B) the lac operon would be maximally repressed since CAP would be inactive and repressor would be uninhibited. With no glucose or lactose (choice D), the level of cAMP would be high so activation of CAP would be evident leading to transcription, but due to the presence of the repressor the rate of transcription would not be maximal.

• Question 495:

  An observation that has been made over the past several decades is that certain dominantly inherited disorders exhibit earlier age of onset along with increasing severity in successive generations. This phenomenon is referred to as anticipation. Amost striking example of this genetic trait is characterized by muscle wasting beginning with the face, neck, and hands gradually becoming generalized and by the inability of muscles to relax after contraction. These symptoms and the phenomenon of anticipation are associated with which of the following diseases?

  A. becker muscular dystrophy (BMD)

  B. DMD

  C. Marfan syndrome

  D. myotonic dystrophy

  E. osteogenesis imperfecta

  Correct Answer: D

  Section: Biochemistry Myotonic dystrophy is the most common muscular dystrophy affecting adults. Apart from the clinical manifestations of the disease described in the question, myotonic dystrophy affects cardiac and smooth muscles, is associated with immune disorder, minor mental retardation, and cataracts. Myotonic dystrophy results from defects in the myotonin gene. These defects are associated with the expansion of a trinucleotide repeat sequence in the 3'- untranslated region of the gene. BMD (choice A) and DMD (choice B) both result from mutations in the DMD gene, but the symptoms of BMD are milder than those of DMD. Transmission of these two diseases is X-linked. Marfan syndrome (choice C) and osteogenesis imperfect are disorders of connective tissue. Marfan syndrome results from defects in the fibrillin gene and osteogenesis imperfecta from defects in collagen. None of the other four diseases exhibit the inheritance phenomenon of anticipation.

• Question 496:

  You are studying the effects of a compound on the respiratory activity of isolated mitochondria. Your experiments demonstrate that oxygen consumption is normal when pyruvate and malate are added. Oxygen consumption is also seen to be normal when succinate is added. However, you find that the production of ATP is severely impaired in the presence of pyruvate/malate or succinate when the compound is added to the reaction. These results most closely resemble the effects that would be seen by the addition of which of the following inhibitors of oxidative phosphorylation?

  A. antimycin A

  B. azide

  C. dinitrophenol

  D. oligomycin

  E. rotenone

  Correct Answer: C

  Section: Biochemistry The normal flow of electrons, through the proteins of the oxidative phosphorylation machinery, is coupled with the establishment of a proton gradient across the inner mitochondrial membrane. The establishment of this pH gradient is the chemiosmotic potential that is coupled with the production of ATP. If the process of proton movement is uncoupled from the normal pathway through the ATP synthase complex, oxygen can be consumed but no ATP will be synthesized. This is the effect of uncoupling agents such as DNP, which act to discharge the proton gradient. Antimycin A (choice A) is an antibiotic that functions by inhibiting cytochrome b of complex III. Azide (choice B) inhibits cytochrome oxidase of complex IV. Oligomycin (choice D), a streptomyces antibiotic, inhibits ATP synthase. Rotenone (choice E) inhibits complex I.

• Question 497:

  The inability to rapidly synthesize DNA during the process of erythrocyte maturation leads to abnormally enlarged erythrocytes. This disorder is referred to as macrocytic anemia and is caused by a deficiency in which of the following vitamins?

  A. ascorbate

  B. biotin

  C. folate

  D. niacin

  E. thiamine

  Correct Answer: C

• Question 498:

  A 12-year-old boy has suffered from chronic sinopulmonary disease including persistent infection of the airway with Pseudomonas aeruginosa. He has constant and chronic sputum production as a result of the airway infection. Additionally, he suffers from gastrointestinal (GI) and nutritional abnormalities that include biliary cirrhosis, meconium ileus, and pancreatic insufficency. The symptoms are classical for which of the following disorders?

  A. congenital adrenal hyperplasia

  B. CF

  C. Renal Fanconi syndrome

  D. sickle cell anemia

  E. Tay-Sachs disease

  Correct Answer: B

• Question 499:

  Exhbit:

  

  Please refer to the exhibit. Hypersensitive individuals have IgE to specific antigens (e.g., pollen, bee venom) on the surface of their leukocytes (monocytes, macrophages, basophils, eosinophils). When these individuals are challenged with antigen, the antigen-IgE complexes induce synthesis and release of which of the following physiologically potent lipids?

  A. A

  B. B

  C. C

  D. D

  E. E

  Correct Answer: C

• Question 500:

  Acetyl-CoA enhances the rate of gluconeogenesis by acting as an allosteric activator of which of the following enzymes?

  A. ACC

  B. PEP-carboxykinase

  C. pyruvate carboxylase

  D. PDH

  E. pyruvate kinase

  Correct Answer: C

  Section: Biochemistry The major substrates of gluconeogenesis are pyruvate and lactate. During gluconeogenesis lactate is oxidized to pyruvate. For pyruvate to be converted back to glucose, it must first be carboxylated to oxaloacetate, since a reversal of the PK reaction of glycolysis cannot occur to convert the pyruvate to phosphoenolpyruvate. The carboxylation of pyruvate is catalyzed by the mitochondrial enzyme pyruvate carboxylase. The activity of pyruvate carboxylase is absolutely dependent on the presence of acetyl-CoA, which allosterically activates the enzyme. Of the enzymes listed, only pyruvate carboxylase and PEPCK (choice B) are involved in gluconeogenesis. ACC (choice A) is involved in fatty acid synthesis and is not regulated by acetyl-CoA. PDH (choice D) is the entry point for pyruvate into the TCA cycle. It is also inhibited by acetyl- CoAvia the acetyl-CoA-mediated activation of PDH kinase, an enzyme that phosphorylatesand inactivates PDH. PK (choice E) is a glycolytic enzyme and is inhibited by acetyl-CoA, not activated by it.