A 4-year-old retarded child is seen by an ophthalmologist because he has developed bilateral visual difficulties. Ocular examination demonstrates bilateral lens dislocations. The ophthalmologist notes that this retarded child also shows developing stigmata similar to those of Marfan syndrome, with tall stature for age, kyphosis, and short trunk. Plasma biochemical studies would be most likely to demonstrate a selective elevation of which of the following substances?

A. Cystathionine
B. Cysteine
C. Methionine
D. Phenylalanine
E. Tyrosine

A? not sure

The correct answer is C. The child has cystathionine synthase deficiency. This enzyme occurs in the pathways involving the metabolism of sulfur-containing amino acids, and normally catalyzes the conversion of homocysteine to cystathionine. When the enzyme is blocked, homocysteine, (which is normally a degradation product of methionine), is remethylated by a salvage pathway, resulting in the resynthesis of methionine. The methionine levels in the plasma are elevated as a result. The clinical features illustrated in the question stem are typical. These patients are also vulnerable to (potentially fatal) thrombotic episodes, particularly in postoperative and postpartum periods.
Cystathionine (choice A) and cysteine (choice B) occur in the pathway after the enzyme blockage, and so are reduced.

Associate abnormalities of phenylalanine (choice D) with phenylketonuria, and associate abnormalities of tyrosine metabolism (choice E) with tyrosinemia, albinism, and alkaptonuria.