Bagheera, a 7-year-old neutered male ragdoll cat is presented for a wellness visit and vaccinations. He was previously part of a breeding program, and the owner reports no abnormalities at home. Bagheera weighs 11.9 lb (5.4 kg) on presentation with a BCS of 5/9. Other than a grade II-III/VI left parasternal systolic heart murmur, physical examination is unremarkable. Heart rhythm is normal but fast at 220 bpm, and femoral pulses are strong and synchronous.
Which of the following is not a common cause for a heart murmur in a cat?