Review Question - QID 212313

A 17-year-old boy is brought to the pediatrician by his mother for fatigue. The patient reports that he was supposed to try out for winter track this year, but he had to quit because his “legs just give up.” He also reports increased difficulty breathing with exercise but denies chest pain or palpitations. He has no chronic medical conditions and takes no medications. He has had no surgeries in the past. The mother reports that he met all his pediatric milestones and is an “average” student. He is up-to-date on all childhood vaccinations, including a recent flu vaccine. His temperature is 98.7°F (37°C), blood pressure is 118/71 mmHg, pulse is 91/min, respirations are 12/min, and oxygen saturation is 98% on room air. On physical examination, there is mild lumbar lordosis. The patient’s thighs appear thin in diameter compared to his lower leg muscles, and he walks on his toes. An electrocardiogram shows 1st-degree atrioventricular nodal block. Which of the following is the most likely cause of the patient’s condition?