B is the correct answer.
A systolic heart murmur may be present in dogs with heartworm disease (Dirofilaria immitis) when there is pulmonary hypertension or worms entangled around the tricuspid valve. The murmur is typically caused by tricuspid regurgitation. However, in cats, the lungs are the primary organ affected, as the relatively low worm burden makes development of pulmonary hypertension and a resultant murmur less likely in cats than in dogs.1-4 Thus, a systolic heart murmur is a rare clinical examination finding.5,6
Hyperthyroidism, a significant cause of functional murmurs in cats, produces changes in cardiac contractility, myocardial oxygen consumption, cardiac output, blood pressure, and systemic vascular resistance.7,8 These changes can lead to turbulent flow in the outflow tracts secondary to increased cardiac output, which can manifest as a murmur during auscultation. Hyperthyroidism can also lead to significant cardiac hypertrophy, primarily through the increased work load on the heart by way of an increase in the hemodynamic load.9 Because these changes can mimic hypertrophic cardiomyopathy (HCM), it is important to check thyroid levels in cats showing echocardiographic evidence of HCM (eg, left ventricular hypertrophy, eccentric hypertrophy of the left ventricle).
Physiologic heart murmurs are relatively common in cats and are typically caused by dynamic right ventricular outflow tract obstruction (DRVOTO),10 a unique physiologic condition in cats in which the flow velocity through the proximal right ventricular outflow tract is increased, possibly due to the comparatively hyperdynamic ventricular systolic function in cats.
The most common causes for heart murmurs in cats are underlying cardiac disease resulting from HCM (specifically, dynamic left ventricular outflow tract obstruction [DLVOTO]) and DRVOTO.10-15 HCM is the most common form of underlying heart disease affecting cats. Other types of heart disease (eg, congenital heart disease, restrictive cardiomyopathy, unclassified cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, dilated cardiomyopathy) are also possible differentials.10,12-19
Hypertension in cats can lead to left ventricular hypertrophy (LVH), subsequent diastolic dysfunction, and development of changes that could result in development of a heart murmur. However, it is important to note that hypertension is not a direct cause of a heart murmur; rather, the secondary effects of sustained systemic hypertension to the left ventricle and valvular structures can lead to LVH and hemodynamic alterations that may result in development of a heart murmur.