Kaplan USMLE Step 1: What is causing teen's reddish urine?

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.

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).

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.