A pediatric urine sample shows many six-sided crystals. The most likely diagnosis is:

• A. Upper urinary tract infection
• B. Maple syrup urine disease
• C. Cystinuria
• D. Glomerulonephritis

Answer: (C)

Hexagonal crystals in pediatric urine are a classic clue for cystinuria. This condition stems from a genetic defect in the renal reabsorption of cystine (and other dibasic amino acids), so cystine builds up in the urine. Cystine is poorly soluble, especially at normal urine pH, and precipitates as distinctive six-sided crystals. Seeing these crystals strongly points to cystinuria as the cause of the crystals and potential stones in a child. In contrast, the other conditions would show different clinical features or crystal shapes (for example, infections with bacteria, a maple syrup odor from metabolic disease, or glomerular symptoms) rather than this hexagonal crystal pattern.