A 9-year-old girl is found to be nearsighted during a vision screening at school, and the school nurse tells the parents the child should be fitted for corrective lenses. The patient is much taller than her classmates, has an awkward gait, and was recently diagnosed with scoliosis.When the ophthalmologist observes that the patient’s right lens is dislocated, he suspects that her symptoms are in fact related to an enzyme deficiency. As a result of this deficiency, which of the following amino acids is essential in this patient’s diet?
The correct answer is C. Homocystinuria is a error of metabolism caused by a defect in cystathionine synthase, the enzyme that converts homocysteine to cystathionine. Cystathionine is later converted to cysteine, so patients with this enzyme deficiency are required to supplement their diets with exogenous cysteine. In addition to marfanlike features and subluxation of the lens, these patients are at increased risk of a variety of cardiovascular derangements, including premature vascular disease and early death.