Free Online USMLE Step 1 Questions and Answers 8

4. A research scientist studying the metabolic pathways that contribute to obesity feeds experimental animals a highcarbohydrate, high-protein diet for a prolonged period. A sample of liver tissue is then obtained from the animals, and the activity of various enzymes involved in fatty acid metabolism is measured and recorded. It is determined that beta-oxidation of fatty acids is inhibited within these cells as a result of the diet. An increase in which of the following substances is most likely responsible for the observed effect?

A. Acetoacetate
B. Camitine
C. Citrate
D. Malonyl-CoA
E. NADPH

Correct Answer: D

Answer Explanation:

(Choice A) Ketone bodies (eg, acetoacetate) are a major source of fuel for muscle, brain, and cardiac tissue that are produced during times of starvation or fasting when oxaloacetate is in short supply and acetyl-CoA is in excess.
(Choice B) Carnitine is an amino acid that is essential for the transport of fatty acids through the mitochondrial membrane. It is a necessary component of fatty acid oxidation.
(Choice C) Citrate is an intermediate in the TCA cycle. It can be exported out of the mitochondrial matrix and into the cytosol, where it is broken down into acetyl-CoA for use in de novo fatty acid synthesis.
(Choice E) NAOPH is a reducing molecule necessary for the synthesis of fatty acids. In contrast, beta-oxidation of fats produces FADH2 and NADH-reducing equivalents.

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5. A 6-year-old girl with chronic anemia requiring repeated blood transfusions is undergoing genetic testing. The patient’s mother and older sibling have a history of mild anemia. Her peripheral blood smear shows hypochromic, microcytic red blood cells, and hemoglobin electrophoresis reveals a predominance of hemoglobins F and A2. Sequencing of the β-globin gene is performed using the patient’s erythroblast DNA. A schematic representation of the gene and its transcribed RNA is shown in the image below.

Usmle8 1

The base sequence indicated by the bold red arrow is responsible for which of the following functions?

A. Enhancement of transcription
B. Initiation of transcription
C. Initiation of translation
D. Repression of transcription
E. Termination of transcription

Correct Answer: B

Answer Explanation:

(Choice A) Enhancer sequences bind activator proteins that facilitate bending of DNA. DNA bending allows activator proteins to interact with general transcription factors and RNA polymerase II at the promoter, increasing the rate of transcription. Enhancers can be located upstream or downstream from the gene being transcribed and may be near the gene or thousands of base pairs away.
(Choice C) Translation is initiated in the cytoplasm when a ribosome recognizes the AUG (methionine) start codon on a mature mRNAstrand. The Kozak consensus sequence plays a major role in initiation of the eukaryotic translation process.
(Choice D) Silencers are similar to enhancers, but they decrease transcription rates by binding repressor proteins.
(Choice E) Eukaryotic transcription termination is not completely understood. In prokaryotes, a palindromic code in the DNA template causes formation of a “hairpin” turn in the newly synthesized mRNA, which faci litates detachment of RNA polymerase from the DNA template.

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6. A 2-year-old boy is evaluated for easy bruising. His parents report that he develops marked bruising and open wounds following minor trauma. The skin is difficult to suture due to its extreme fragility. Physical examination reveals hyperextensible skin, multiple ecchymoses over the forearms and pretibial regions, and an umbilical hernia. A skin biopsy is performed, and histochemical evaluation of the biopsy reveals a defect in extracellular processing of collagen. Which of the following steps of collagen synthesis is most likely impaired in this patient?

A. Glycosylation of hydroxylysine residues
B. lnterchain C-terminal disulfide bond formation
C. N-terminal propeptide removal
D. Proline residue hydroxylation
E. Triple helix formation

Correct Answer: C

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