A 16 year old boy comes to you for a routine sports physical. He has no symptoms and is feeling pretty well. An adolescent HEADSS interview reveals that his parents are divorced but he gets along with both, he is a B+ student, he enjoys running and playing video games, he has tried alcohol but no other drugs, he is sexually active with one partner, and he has no suicidal thoughts. He has no past medical history and takes no medications. His family history reveals that a paternal grandfather died of heart failure at age 50, an uncle died of sudden death at age 25, and his father has unexplained dyspnea on exertion and chest pain though a recent coronary arteriogram was normal.
On exam, the patient's vitals are normal and he is in no acute distress. On cardiac exam, you feel a prominent left ventricular lift with a diffuse apical impulse. You hear an S4 along with a harsh crescendo-decrescendo systolic murmur best heard at the apex and left lower sternal border. The murmur radiates to the axilla and base. When you have the patient Valsalva, the murmur gets louder. When you elevate the patient's legs, the murmur gets softer. The carotid pulses are normal. The lungs are clear to auscultation bilaterally. The abdomen is benign. There is no cyanosis, clubbing, or edema. An EKG is taken in the office and shown below.
Challenge: What's the most likely diagnosis?
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