USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 481:

  If the RNA synthesized in Question 398 were translated in a eukaryotic in vitro translation system, what would be the composition of the resultant peptide?

  

  A. C-T-I-P-Y

  B. H-S-I-A-C

  C. M-L-W-N

  D. T-C-Y-G-M

  E. V-R-Y-R-T

  Correct Answer: C

  Section: Biochemistry Translation in a eukaryotic in vitro translation system would begin at an AUG codon residing near the 5'end of the mRNA. Therefore, translation of the correct RNA product would begin two nucleotides from the 5' end at the first AUG codon. This would result in the translation of a protein of four amino acids. None of the other choices (A, B, D, and E) translate into a four-unit amino acid chain.

• Question 482:

  Which of the following statements most correctly reflects the effect of epinephrine stimulation of adipocytes?

  A. enzyme-dependent addition of fatty acids to glycerol-3-phosphate

  B. enzyme-dependent stepwise release of fatty acids from triglycerides

  C. increased synthesis and activation of LPL

  D. increased synthesis of HDL to transport fatty acids to peripheral tissues E. release of glycerol-3-phosphate for gluconeogenesis in the liver

  Correct Answer: B

  Section: Biochemistry When epinephrine stimulates adipocytes (by interaction with beta-adrenergic receptors), the immediate response is the activation of adenylate cyclase, which in turn leads to an increase in cAMP. Increased levels of cAMP lead to activation of PKA, which in adipocytes will phosphorylate and activate hormonesensitive lipase. When hormone-sensitive lipase is active, it removes fatty acids in a stepwise manner from stored triacylglycerides. The released fatty acids then enter the blood and are bound by albumin for transport to peripheral tissues including the liver. LPL (choice C) is present on the surfaces of vascular endothelial cells and is activated by apo CII present in VLDLs and chylomicrons. Epinephrine is a stimulus that leads to energy consumption and therefore would not lead to an increase in triacylglyceride synthesis (choice A). When triacylglyceride breakdown is activated in adipocytes, the glycerol backbone is released as free glycerol, not glycerol-3-phosphate (choice E) to the blood and delivered to the liver for gluconeogenesis. Fatty acids released from adipose tissue are transported in the blood bound to albumin, not within any lipoprotein particle (choice D).

• Question 483:

  If the DNA strand shown below is used as a template for RNA polymerase, what would be the sequence of

  the resultant mRNA following transcription?

  5'-CATTCCATAGCATGT-3'

  A. 5'-ACAUGCUAUGGAAUG-3'

  B. 5'-CAUUCCAUAGCAUGU-3'

  C. 5'-GUAAGGUAUCGUACA-3'

  D. 5'-UGUACGAUACCUUAC-3'

  Correct Answer: A

  Section: Biochemistry Transcription occurs in the 5'3' direction, and during this process RNA polymerase will move in the 3'5' direction relative to the template DNA strand. Therefore, the correct transcriptional product from the DNA template begins with its 5' ribonucleotide corresponding to the complementary deoxyribonucleotide of the 3' end of the template. None of the other RNA strands (choices B, C, D, and E) could be products generated from the template shown.

• Question 484:

  Following a relatively normal early developmental period, a 6-month-old boy becomes pale and lethargic and begins to show signs of deteriorating motor skills. The infant has severe megaloblastic anemia; however, serum measurements of iron, folate, vitamins and demonstrate they are

  

  within normal range. Urine samples were clear when fresh, but when left to stand for several hours showed an abundant white precipitate that was composed of fine needle-shaped crystals. Analysis of the crystals identified them as orotic acid. Significant improvement is observed in the infant following oral administration of a nucleoside. Which of the following is most likely the nucleoside used?

  A. adenosine

  B. cytidine

  C. guanosine

  D. thymidine

  E. uridine

  Correct Answer: E

  Section: Biochemistry Hereditary orotic aciduria results from a defect in the de novo synthesis of pyrimidines. The defect is in the bifunctional enzyme that catalyzes the last two steps in the de novo pathway, conversion of orotic acid to OMP and OMP to UMP. Administration of uridine allows afflicted individuals to produce sufficient levels of cytidine nucleotides via the salvage pathways. Treatment with uridine leads to a return of normal blood hemoglobin levels, and bone marrow will become normoblastic. Treatment with cytidine (choice B) has some limited ability to ameliorate symptoms of the disease, but not to the extent of uridine administration. None of the other nucleosides (choices A, C, and D) can be salvaged into cytidine or uridine nucleotides, and are therefore of no clinical value in the treatment of hereditary orotic aciduria.

• Question 485:

  Numerous cancers are caused by a genetic phenomenon termed "loss of heterozygosity," LOH. This phenomenon led to the identification of genes termed tumor suppressors, because it is the loss of their function that leads to cancer. Which of the following has been shown to result from defects in a tumor suppressor gene as a consequence of LOH?

  A. Creutzfeldt-Jakob disease (CJD)

  B. Crouzon syndrome

  C. HD

  D. Li-Fraumeni syndrome (LFS)

  E. Prader-Willi syndrome

  Correct Answer: D

  Section: Biochemistry LFS is a rare inherited form of cancer that involves breast and colon carcinomas, softtissue sarcomas, osteosarcomas, brain tumors, leukemia, and adrenocortical carcinomas. These tumors develop at an early age in LFS patients. The tumor suppressor gene found responsible for LFS is p53. Mutant forms of p53 are found in approximately 50% of all tumors. The normal p53 protein functions as a transcription factor that can induce either cell-cycle arrest or apoptosis (programmed cell death) in response to DNA damage. CJD (choice A) encompasses three forms: infectious, sporadic, and inherited, with the vast majority of cases being sporadic. Clinical abnormalities of CJD are confined to the CNS and result from a pathogenic protein identified as prion protein (PrP). Crouzon syndrome (choice B) is haracterized by craniosynostosis (midface hypoplasia and ocular proptosis) and is the result of a mutation in one of the receptors for fibroblast growth factor (FGFR2). HD (choice C) is an autosomaldominant disorder resulting from expansion of the triplet CAG within the huntingtin gene. The exact function of the huntingtin protein is still unclear. Symptoms of HD include personality changes, memory loss, and involuntary leg and arm movements (chorea). The average age of onset is 37 years. PWS (choice E) is a relatively common cause of genetic obesity and mental retardation. Symptoms of severe hypotonia and poor suckling are evident at birth. PWS is caused by a deletion of a portion of the long arm of chromosome 15 [del(15q11q13)].

• Question 486:

  In analyzing a sample of double-stranded DNA, it has been determined that the molar ratio of adenosine is 20%. Given this information, what is the content of cytidine?

  A. 10

  B. 20

  C. 30

  D. 40

  E. 60

  Correct Answer: C

  Section: Biochemistry The molar ratio of adenosine in any molecule of double-stranded DNA will be equivalent to that of thymidine, since these two nucleotides hydrogen bond to form base pairs. Therefore, the total amount of the DNA accounted for in A-T base pairs would be 40%. Since guanosine and cytidine hydrogen bond to form base pairs, they too contribute an equivalent molar ratio in double-stranded DNA. In double-stranded DNA with 40% A-T composition, the molar ratio of cytidine would be half of the remaining 60% of the DNA, or 30%. No other molar ratio (choices A, B, D, and E) could account for the amount of cytidine in a double-stranded DNA with 20% adenosine.

• Question 487:

  Measurement of the rate of creatinine clearance is used as a key determinant of renal function. In which of the following tissues is creatinine generated?

  A. adipose

  B. kidney

  C. liver

  D. lung

  E. skeletal muscle

  Correct Answer: E

  Section: Biochemistry In cardiac and skeletal muscle, high-energy phosphate is stored through the transfer of a phosphate from ATP to creatine-generating creatine phosphate. Creatine is synthesized in liver from guanidoacetate. Guanidoacetate is derived from the amino acid arginine in the kidneys (below figure). In muscle cells, creatinine is a nonenzymatic metabolite of creatine phosphate. When measured in the serum, levels of creatinine are remarkably constant from day to day and are proportional to muscle mass. In renal dysfunction, the clearance of creatinine will be impaired and its levels will therefore rise in the serum. Although creatine is synthesized in the liver (choice C) from guanidoacetate, which is produced in the kidneys (choice B), it is not used by these two tissues, nor by the lung (choice D) or adipose tissue (choice A). Once synthesized, creatine is transported to cardiac and skeletal muscle where it is phosphorylated and stored for future energy needs.

• Question 488:

  A 15-year-old boy has been diagnosed with retinitis pigmentosa, peripheral polyneuropathy, and cerebellar ataxia. Analysis of his cerebrospinal fluid indicated a high protein content but no elevation of cell number. A telling clinical finding was a high level of serum phytanic acid. These findings are indicative of which of the following disorders?

  A. carnitine palmitoyltransferase I deficiency

  B. fragile X syndrome

  C. MCAD deficiency

  D. Refsum disease

  E. rhizomelic chondrodysplasia punctata

  Correct Answer: D

  Section: Biochemistry Refsum disease is the result of defects in the oxidation of phytanic acid, a lipid requiring an alpha-oxidation pathway. As a consequence, phytanic acid accumulates in the blood and tissues. The hallmark symptoms of the disease are retinitis pigmentosa, cerebellar ataxia, chronic polyneuropathy, and an elevation in protein in the cerebrospinal fluid with no increase in cell count. Adeficiency in carnitine palmitoyltransferase I (choice A) primarily affects the liver and results in reduced fatty acid oxidation and ketogenesis. Fragile X syndrome (choice B) is the most common form of heritable mental retardation worldwide. MCAD deficiency (choice C) is one of most frequently inherited defects in metabolism. Clinical crisis is characterized by an infant presenting with episodes of vomiting and lethargy, seizures, and coma associated with a period of fasting. On presentation, the infant may be comatose and blood glucose will be low with low to no ketones in urine. RCDP (choice E) is caused by a disorder in peroxisome biogenesis. Rhizomelia refers to striking shortening of the proximal limbs. Patients also have severely disturbed endochondral bone formation. In addition, RCDP patients have profound psychomotor retardation.

• Question 489:

  The occurrence of Type II diabetes (NIDDM) in adolescent females can lead to the development of polycystic ovarian syndrome (PCOS). PCOS is the result of follicular atresis and ovulatory dysfunction brought about by a hyperandrogenic microenvironment in the ovary. Which of the following statements reflects the underlying cause of the hyperandrogenic state in females with NIDDM?

  A. Hyperinsulinemia, associated with NIDDM, reduces the level of sex hormone-binding globulin leading to increased free testosterone.

  B. NIDDM in adolescents is primarily the result of obesity and the associated disruption in fatty acid metabolism negatively affects adrenal estrogen production.

  C. The increased level of circulating lipid in NIDDM patients competes for steroid binding to sex hormone-binding globulin, resulting in a reduced transport of estrogen within the ovary.

  D. The persistent hyperglycemia associated with NIDDM causes increased levels of glycosylated hemoglobin, which interferes with the need for increased ovarian vascularization at puberty.

  Correct Answer: A

  Section: Biochemistry Adolescent females with Type II diabetes have a high occurrence of PCOS, which is brought on by the hyperinsulinemia associated with Type II diabetes. Insulin effects on the ovary drive conversion of progesterone to testosterone and a reduction in serum hormonebinding globulin (SHBG). Taken together, the effects of hyperinsulinemia lead to a hyperandrogenic state in the ovary resulting in follicular atresis and ovulatory dysfunction. None of the other options (choices BD) are associated effects caused by Type II diabetes leading to PCOS.

• Question 490:

  When the body becomes acidotic, the liver and the kidneys play central roles in reactions designed to reduce the proton level (i.e., increase the pH) of the circulation. Which of the following constitutes the hepatic reaction occuring during periods of acidosis?

  A. ammonia incorporation into glutamateforming glutamine

  B. ammonia incorporation into alphaketoglutarate- forming glutamate

  C. glutamine conversion to glutamatereleasing ammonia

  D. glutamine conversion to alphaketoglutarate- releasing ammonia

  E. increased production of urea to dispose of ammonia

  Correct Answer: A