USMLE Q Bank Step 1- 204

A 42 year-old woman presents to the clinic with progressive fatigue and weakness for 6 months. She also complains of tingling and numbness of her arms and legs. Physical examination reveals decreased both superficial and deep sensations in her arms and legs. Blood count reveals decreased numbers of erythrocytes, leucocytes and platelets along with increased mean red blood cell volume. Peripheral blood smear shows enlarged erythrocytes and hypersegmented neutrophils.

Which one of these biochemical reactions is involved in this case?

A- Conversion of propionyl –CoA to methylmalonyl –CoA

B- Conversion of methylmalonyl-CoA to succinyl-CoA

C- Conversion of succinyl-CoA to fumarate

D- Conversion of malonyl-CoA to palmitate


CBC reveals megalocytosis and hypersegmented neutrophils which are the main characteristics of megaloblastic anemia. The most common causes of megaloblastic anemia are Folic acid deficiency and vitamin B12 deficiency. Vitamin B12 deficiency anemia is differentiated from folic acid deficiency anemia by neurological symptoms in the from of sensory loss. Vitamin B12 in involved in conversion of methylmalonyl CoA into succinyl CoA. This is an important step in the formation of DNA. Deficiency of vitamin B12 leads to impaired DNA production and subsequently megaloblastic anemia. Other reactions are not involved in B12 deficiency.

The correct answer is B

2 thoughts on “USMLE Q Bank Step 1- 204”

  1. Thank you Dr Abhishek Karn

  2. B

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