This is the answer and explanation for a USMLE practice question from the Dr. Ray Makar Facebook page. Try it first, then check your answer below.
The Question
A 24-year-old woman of Mediterranean descent has mild anemia: Hb 10.9 g/dL, MCV 64 fL and a high RBC count of 5.8 million/mm3. Ferritin and iron are normal, and iron supplements did not help. Which finding is most likely on further testing?
A. Decreased serum ferritin
B. Increased hemoglobin A2 on electrophoresis
C. Ringed sideroblasts in the bone marrow
D. Hemoglobin S on electrophoresis
E. Coarse basophilic stippling from lead
Correct Answer: B. Increased hemoglobin A2 on electrophoresis
This is beta-thalassemia minor (trait). Clues: Mediterranean background, very low MCV with only mild anemia, a high RBC count and normal iron studies (Mentzer index MCV/RBC = 11, below 13). Hemoglobin electrophoresis shows increased HbA2 (and sometimes HbF) because less beta chain is made.
Why the Other Answers Are Wrong
- Decreased ferritin: Iron deficiency, but her ferritin is normal and iron did not help.
- Ringed sideroblasts: Sideroblastic anemia (alcohol, lead, B6 deficiency), usually with high iron.
- Hemoglobin S: Sickle cell disease or trait, not a cause of marked microcytosis like this.
- Basophilic stippling from lead: Lead poisoning, which has other clues (abdominal pain, neuropathy, exposure).
High-Yield Pearl
Microcytosis out of proportion to the anemia + high RBC count + normal iron = thalassemia trait. Beta = high HbA2.
Want more questions like this? Get Pathology MCQs For Medical Students on Amazon.
More free practice: all practice questions