Homocystinuria is an inherited metabolic disorder — most commonly from deficiency of the enzyme cystathionine beta-synthase — that prevents the normal conversion of the amino acid methionine to cysteine via homocysteine. Homocysteine accumulates in blood and urine to toxic levels, damaging blood vessels (predisposing to early strokes and clots), the eyes (lens dislocation), the skeleton (tall, Marfan-like build), and the brain (developmental delay). Detected by newborn screening in many countries, it is managed with a methionine-restricted diet, vitamin B6 (which boosts residual enzyme activity), betaine, and folate. Early, strict treatment prevents most complications — another victory for screening and dietary medicine over a devastating genetic lottery.