You are answering a USMLE-style medical question. Question: A 30-year-old male presents to his primary care physician complaining of infertility. He and his wife have been trying to get pregnant for the past two years. They have used fertility monitors and other aids without success. A hysterosalpingogram in his wife was normal. The patient has a history of cleft lip and recurrent upper respiratory infections as a child. He was briefly hospitalized for severe pneumonia when he was 9-years-old. His temperature is 98.6°F (37°C), blood pressure is 120/85 mmHg, pulse is 90/min, and respirations are 18/min. On examination, he is a healthy-appearing male in no acute distress with fully developed reproductive organs. Notably, cardiac auscultation is silent in the left 5th intercostal space at the midclavicular line. This patient most likely has a mutation in which of the following classes of proteins? Options: A) Microtubule monomeric protein B) Transmembrane ion channel protein C) Retrograde cytoskeletal motor protein D) Anterograde cytoskeletal motor protein Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.