**Core Concept**
A double apical impulse is a physical examination finding associated with abnormalities in left ventricular contractility and relaxation. It is a sign of a condition where the left ventricle contracts and relaxes in an abnormal manner, leading to an extra beat or impulse felt at the apex.

**Why the Correct Answer is Right**
In hypertrophic obstructive cardiomyopathy (HOCM), the left ventricle becomes thickened, leading to a narrowing of the outflow tract. This narrowing causes a dynamic obstruction to blood flow, which can result in a double apical impulse. The extra impulse is due to the obstruction causing a premature beat or a delayed relaxation of the ventricle. This is often accompanied by other signs such as a systolic ejection murmur and a pulsus bisferiens.

**Why Each Wrong Option is Incorrect**
**Option A:** Aortic stenosis (AS) and aortic regurgitation (AR) may have a double apical impulse, but it is not a characteristic finding. The primary signs of AS are a systolic ejection murmur and a diminished pulse pressure, while AR is characterized by a diastolic murmur and a wide pulse pressure.

**Option B:** Tricuspid regurgitation (TR) and tricuspid stenosis (TS) do not typically cause a double apical impulse. TR is characterized by a diastolic murmur and a wide pulse pressure, while TS has a low-pitched, rumbling diastolic murmur.

**Option C:** Myocardial infarction (MI) may cause a variety of physical examination findings, including a new systolic murmur or a pericardial friction rub, but a double apical impulse is not a characteristic sign.

**Clinical Pearl / High-Yield Fact**
In patients with HOCM, a double apical impulse may be accompanied by a characteristic "square root of P" sign on the ECG, indicating a prolonged PR interval and a narrow QRS complex.

**✓ Correct Answer: D. HOCM**