Cystathioninuria is a rare inherited disorder of sulfur amino acid metabolism in which the body cannot efficiently convert methionine to cysteine. The block sits at the enzyme cystathionine beta-synthase pathway, so the intermediate cystathionine builds up and spills into the urine in large amounts. Many people with the condition never notice symptoms — it is often found by chance on a urine screening — but the underlying disruption of methionine handling can matter for growth and development, and severe forms have been linked with developmental delay. Nutritionally, the condition is a reminder of how tightly the sulfur amino acids are linked: methionine is essential and must come from protein foods, while cysteine can normally be made from it. Management, where needed, centres on monitoring amino acid status and adjusting protein intake under metabolic supervision.