A 10-year-old girl is brought to her pediatrician for a routine examination. The child is thin and has long fingers and toes, dislocated lenses, and mental retardation. Enzyme studies show an inability to convert homocysteine to cystathionine by cystathionine b-synthase. Newborn screening would have been able to identify the metabolic disorder present in this girl through the measurement of an increased serum concentration of which of the following amino acids?

Question

Quizplus · Accepted Answer

The inability to convert homocysteine to cystathionine due to cystathionine b-synthase deficiency leads to homocystinuria, which is characterized by elevated levels of methionine in the blood. The most common causes of homocystinuria are defects in the cystathionine b-synthase (CBS) gene. CBS is involved in the conversion of methionine to cysteine and so a deficiency in the enzyme can be suspected in cases where there are elevated levels of methionine in the blood. Homocystinuria is often associated with mental retardation, although the complete syndrome is multifaceted and many individuals with this disease are mentally normal, while others experience variable levels of developmental delay along with learning problems. Common symptoms of homocystinuria are dislocated optic lenses, osteoporosis, lengthening and thinning of the long bones, and an increased risk of abnormal blood clotting (thromboembolism).

A 10-Year-Old Girl Is Brought to Her Pediatrician for a Routine