USMLE Step 1 Question 389

Question:

A 46-year-old man goes on a camping trip and ingests undercooked pike Fish from a nearby lake. Eleven months later, he complains of progressive weakness and fatigue. His CBC reveals a hemoglobin of 9.6 g/dL. What is the likely cause of his anemia?

Correct Answer: E.  Vitamin B12 deficiency

This man probably ingested larvae of Diphyllobothrium latum from raw freshwater fish. This fish tapeworm causes few abdominal symptoms, but can absorb vitamin B12 (choice E) to a great degree, causing a deficiency. This deficiency causes a macrocytic anemia.

Folate deficiency (choice A) can also cause a macrocytic anemia. Iron deficiency (choice B) and pyridoxine deficiency (choice D) cause a microcytic anemia.

Pyridoxine deficiency can cause anemia. But, there are neurological symptoms secondary to poor nerve conduction, especially in the hands and feet and it would be unlikely to be caused by the events in the vignette.