Q. A 35-year-old male is brought to the emergency department after he fainted while shoveling snow out of his driveway. According to his wife, he lost consciousness for approximately 1 to 2 minutes after which he rapidly regained consciousness. The patient did not experience any drowsiness or memory loss after his episode. Recently, the patient has been experiencing shortness of breath and chest discomfort while climbing the stairs at home. He does not use tobacco, consumes alcohol or use illicit drugs. On examination, blood pressure is 130/80 mm Hg, heart rate 76/min, and respiration rate 14/min. Physical examination and chest x-ray are normal. If the chest examination reveals a crescendo-decrescendo systolic murmur on left lower sternal border without radiation towards the carotids, what is the most probable diagnosis?