You are answering a USMLE-style medical question. Question: A 1-year-old boy is brought to the clinic by his parents for a regular check-up. His weight, height, and head size were found to be in the lower percentile ranges on standard growth curves. His hair is tangled and has a dry, brittle texture. Genetic testing reveals that the patient has a connective tissue disorder caused by impaired copper absorption and transport. The patient’s disorder is caused by a mutation in which of the following genes? Options: A) ATP7A B) COL5A1 C) FBN1 D) ATP7B Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.