A 29-year-old man is evaluated following a syncopal episode in his office for approximately 60 seconds. There was no prodrome of dizziness, palpitations, nausea, or chest pain. He has had no previous similar episodes; He has noticed exertional dyspnea and occasional palpitations for the last 3 years. He takes no medications. On examination, HR 50/min and BP 125/75 mm Hg with no orthostatic changes. There is no jugular venous distention, with normal carotid upstrokes. Lungs are clear. Cardiac examination shows a grade 3/6 systolic murmur at the left sternal border that increases with the Valsalva maneuver. A soft S4 is present, ECG shows sinus bradycardia, first- degree atrioventricular block, and left ventricular hypertrophy. Which of the following is the most likely cardiac diagnosis in this patient?