A 5-year-old girl with moderate mental retardation is brought to the physician by her mother who is concerned that her daughter has been having difficulty with her vision for weeks. Physical examination shows a fair complexion and malar erythema. Ophthalmologic examination shows dislocated lenses bilaterally. She has long, slender hands and fingers. Blood tests reveal her cystathionine concentration is below normal. This patient is most likely to have an increased urine concentration of which of the following amino acids?

Question

Quizplus · Accepted Answer

The symptoms and lab findings suggest homocystinuria, a condition often due to cystathionine beta-synthase deficiency, leading to elevated methionine and homocysteine levels in the blood and urine.  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.

A 5-Year-Old Girl with Moderate Mental Retardation Is Brought to the Physician