What condition is associated with the buildup of methionine and homocysteine in plasma?

What condition is associated with the buildup of methionine and homocysteine in plasma?

• A. Alkaptonuria
• B. Homocystinuria
• C. Cystinuria
• D. Tyrosinemia

Answer: (B)

The condition characterized by the accumulation of methionine and homocysteine in plasma is homocystinuria. This metabolic disorder arises from a deficiency in enzymes responsible for the metabolism of homocysteine, primarily cystathionine beta-synthase or, less commonly, other enzymes involved in the methionine cycle.

In homocystinuria, the lack of adequate enzyme activity leads to the inability to convert homocysteine into cystathionine and subsequently into cysteine or recycle it back to methionine, resulting in increased concentrations of both homocysteine and methionine in the blood. This accumulation can lead to various clinical manifestations, including cardiovascular issues, skeletal abnormalities, and developmental delays, among others.

The other conditions mentioned involve different metabolic pathways and do not directly cause the characteristic elevations of homocysteine and methionine seen in homocystinuria. For example, alkaptonuria involves the buildup of homogentisic acid, cystinuria is related to the defective renal transport of certain amino acids including cystine, and tyrosinemia involves issues in the breakdown of tyrosine. Thus, the specific enzyme deficiency seen in homocystinuria is what links the