<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 4-year-old boy presents to the opthalmologist for a down- and inward dislocation of the lens in his left eye. On physical exam, the boy has a marfanoid habitus and mental retardation. Biochemical tests were performed to locate the exact defect in this boy. It was found that there was a significant reduction of the conversion of 5,10-methyltetrahydrofolate to 5-methyltetrahydrofolate. Which of the following is the diagnosis?? A: Marfan syndrome, B: Homocystinuria, C: Alkaptonuria, D: Phenylketonuria, E: Maple syrup disease,  <response>: This is a clinical vignette describing a 4-year-old boy with lens dislocation, marfanoid habitus, mental retardation, and reduced conversion of 5,10-methyltetrahydrofolate to 5-methyltetrahydrofolate. The reduced conversion of 5,10-methyltetrahydrofolate to 5-methyltetrahydrofolate indicates a defect in the methionine cycle, specifically in the enzyme methylenetetrahydrofolate reductase MTHFR. This results in elevated homocysteine levels and decreased methionine levels, which is characteristic of homocystinuria.Marfan syndrome can cause lens dislocation and marfanoid habitus but does not cause mental retardation or abnormalities in the methionine cycle. The other answer choices describe unrelated metabolic disorders that do not fit the clinical and biochemical findings described.Therefore, the diagnosis is homocystinuria.B: Homocystinuria 