You are answering a USMLE-style medical question. Question: A 5-year-old female is brought to a speech therapist for continuing work on improving her communication skills. She is only able to verbalize two word sentences and has generalized developmental delay. When she was born it was noticed that she had a high pitched mewing cry and subsequent physical exam revealed microcephaly, prominent epicanthal folds, and a holosystolic murmur best heard in the left 5th intercostal space near the sternum. An abnormality of which of the following chromosomes is most likely responsible for this patient's disorder? Options: A) 5 B) 7 C) 18 D) 21 Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.