Free Online USMLE Step 1 Questions and Answers 4


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4. A 20-year-old woman comes to the clinic for evaluation of fatigue that has progressively worsened over the past month. The patient was recently  diagnosed with celiac disease, but she has not strictly adhered to a gluten-free diet. Laboratory evaluation reveals macrocytic anemia with a low folate level but normal vitamin 8 12 level. Oral folic acid is prescribed. Which of the following biochemical changes is most likely to occur in this patient after starting treatment?

Usmle4 3

Correct Answer: A

Answer Explanation:

(Choice B) Decreased homocysteine, increased methionine, and decreased methylmalonic acid would be expected after vitamin B12 supplementation, not folate supplementation.
(Choice D) Increased homocysteine, decreased methionine, and increased methylmalonic acid would be expected in vitamin B12 deficiency.
(Choice E) Isolated methylmalonic acidemia occurs in a rare autosomal recessive condition and is unrelated to folate deficiency and supplementation.

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5. A 52-year-old man is being evaluated in the emergency department for abdominal pain associated with watery diarrhea. His symptoms have been progressive over the last month. He says that he is depressed and often has difficulty remembering things. The patient has a 20-year history of alcohol use disorder. On examination, he appears disheveled. A pigmented scaly skin rash is present in the malar distribution of his face, neck, and back of his hands. The rash has been present for several months and worsens on exposure to sunlight. It is determined that the patient’s symptoms are secondary to lack of a specific nutrient. Which of the following enzymes is most likely to be directly affected by this patient’s nutrient deficiency?

A. Citrate synthase
B. Hexokinase
C. lsocitrate dehydrogenase
D. Phosphoglycerate kinase
E. Succinate dehydrogenase

Correct Answer: C

Answer Explanation:

(Choice A) Citrate synthase is an enzyme of the citric acid cycle; it does not require NAO or NADP as a cofactor.
(Choices Band D) Hexokinase and phosphogly cerate kinase are enzymes used in glycolysis; they do not require NAO or NADP as cofactors.
(Choice E) Succinate dehydrogenase is an enzyme of the citric acid cycle; it catalyzes the conversion of succinate to fumarate using flavin adenine dinucleotide (FAD) as a cofactor.

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6. Biologists investigating the morphologic changes associated with reversible cellular injury perform a procedure on anesthetized mice to assess the effects of transient hepatic ischemia. During the experiment, they clamp the hepatic artery and obtain liver biopsy samples at varying intervals. The samples are then examined by electron microscopy. Cells that are exposed to longer ischemic periods are found to have reduced numbers of ribosomes attached to the endoplasmic reticulum. This structural change is most likely to impair which of the following cellular functions?

A. ATP production
B. Drug detoxification
C. Synthesis of cell membrane proteins
D. Synthesis of cytosolic proteins
E. Synthesis of steroid hormones

Correct Answer: C

Answer Explanation:

(Choice A) ATP is produced in the cytosol during glycolysis and in the mitochondria during oxidative phosphorylation. The proteins in both of these cellular compartments are synthesized by free ribosomes in the cytosol.
(Choices Band E) Steroid hormone synthesis and drug detoxification are performed by various proteins found within the smooth ER (SER). The SER does not bind to ribosomes as it lacks the translocon complex.

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