Paediatricsmiddle twist
A 16-year-old netballer attends for a pre-participation examination. She is asymptomatic. The femoral pulses are strong and synchronous with the radial pulses. Her arm span equals her height and the palate is normally arched. A grade 2 systolic murmur is heard at the left sternal edge; it becomes louder when she stands up from a squat and softer when she squats again. The second heart sound splits normally with inspiration. Her father's brother died suddenly at 41 while jogging and the autopsy found no cause. Which one of the following conditions does the behaviour of the murmur most strongly suggest?