Final answer:
Homocystinuria is a condition where the body cannot convert homocysteine to cystathionine due to a defect in cystathionine synthase, making cysteine an essential dietary amino acid.
Step-by-step explanation:
Homocystinuria is a metabolic disorder characterized by a defect in the enzyme cystathionine synthase, resulting in the inability to convert homocysteine into cystathionine. Consequently, the amino acid cysteine becomes essential in the diet, as the body can no longer synthesize it sufficiently. Individuals with homocystinuria must thus obtain cysteine from their diet.
The metabolic disorder impacts sulfur-containing amino acids metabolism, and without dietary supplementation of cysteine, serious health problems may arise. Foods rich in cysteine, such as poultry, oats, wheat germ, or dairy products, would be essential for someone living with this condition.