USMLE Step 1 Question 326

Question:

A 86-year-old female patient is being evaluated for fatigue, altered mental status and ataxia at her physician’s office. Physical exam is notable for loss of proprioception, and laboratory values are notable for Hb of 8.9 g/dl with a MCV of 103. A blood smear is notable for macrocytic erythrocytes and hypersegmented neutrophils. Which of the following will most likely be elevated in the blood?

Correct Answer: E.  Methylmalonyl-CoA

This patient is suffering from B12 deficiency which is manifesting as macrocytic anemia with mental status changes and ataxia due to lack of proprioception. B12 is a necessary cofactor in the conversion of methylmalonyl Co-A into Succinyl-CoA (entering the Kreb cycle) and therefore methylmalonyl CoA will build up.

Ferritin is increased in anemia of chronic disease, but there is no sign of a chronic disease in this patient and anemia of chronic disease is usually normocytic HbS is seen in sickle cell disease which would be rare in an 86 year old and a smear would most likely show sickle cells LDH is elevated in hemolytic anemia which is another type of normocytic anemia Methionine is the compound that is regenerated from homecysteine via B12 and THF (active folate). While homocysteine would be elevated, methionine would be decreased in B12 deficiency (and folate deficiency).