You are answering a USMLE-style medical question. Question: A 27-year-old man and his wife come to the physician for preconception genetic counseling. The patient and his wife are asymptomatic and have no history of serious illness. The patient's older brother died at age 13 from heart failure and his 6-year-old nephew currently has severe muscle weakness. Genetic testing in the patient's family shows that his mother is not a carrier of the mutation that is responsible for the disease in his relatives. A pedigree chart of the patient's family is shown. Which of the following is the most likely explanation for the patient not being affected? Options: A) Variable expressivity B) Maternal uniparental disomy C) Gonadal mosaicism D) Incomplete penetrance Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.