Metabolism USMLE Step 1 Practice Question
A 63 year old woman presents with fatigue, numbness in her feet, and difficulty walking. Examination shows decreased vibration sense in both legs and a smooth beefy red tongue. Laboratory studies show hemoglobin 8.9 g/dL, MCV 112 fL, elevated homocysteine, and elevated methylmalonic acid. Which of the following reactions is most directly impaired?
Answer choices
- APyruvate to acetyl CoA
- BPhenylalanine to tyrosine
- CGlucose 6 phosphate to 6 phosphogluconate
- DHomocysteine to methionine and methylmalonyl CoA to succinyl CoACorrect answer
- ETryptophan to niacin
- FMethylmalonyl CoA to propionyl CoA
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