If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
A 17-year-old boy is brought to the physician because of reddish urine. He recently had a sinus infection and received trimethoprim-sulfamethoxazole. Laboratory studies show:
- Erythrocyte count: 4.0 million/mm3
- Hemoglobin: 12.5 g/dL
- Mean corpuscular volume: 90 μm3
- Total bilirubin: 1.8 mg/dL
- Direct bilirubin: 0.8 mg/dL
- Lactate dehydrogenase: 180 U/L
Haptoglobin is decreased. Urinalysis shows 3+ blood, 3+ urobilinogen, negative bilirubin, proteins, leukocyte esterase and nitrite. Urine microscopy shows no red cells. Numerous red blood cells with distinct inclusions are seen on a peripheral blood smear. Which of the following best describes the inclusions seen on peripheral blood smear?
A. Denatured hemoglobin
B. Hemoglobin crystals
C. Iron-containing granules
D. Nuclear remnants
E. Ring-shaped intracellular organisms
The correct answer is A.
Kaplan Medical explains why
Denatured hemoglobin inclusions (Heinz bodies) are seen in red blood cells of patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency in the setting of hemolysis due to recent illness or exposure to certain medications (e.g., sulfonamide antibiotics).
Why the other answers are wrong
Choice B. Hemoglobin crystals are collections of precipitated hemoglobin C that are seen in red blood cells of patients with hemoglobin C or hemoglobin SC. Although hemolytic anemia is associated with these hemoglobinopathies, hemolysis in response to sulfonamide antibiotic is more characteristic of G6PD deficiency.
