**Core Concept**
Hyperophic obstructive cardiomyopathy (HOCM) is a condition characterized by abnormal thickening of the heart muscle, leading to obstruction of blood flow. In HOCM, the heart muscle becomes stiff and non-compliant, making it difficult for the heart to relax and fill with blood during diastole.

**Why the Correct Answer is Right**
Beta blockers, such as propranolol and metoprolol, are contraindicated in HOCM because they can worsen the obstruction by further decreasing the heart rate and contractility, leading to increased outflow tract gradients. This is because beta blockers reduce the force of contraction and slow the heart rate, which can exacerbate the obstruction in HOCM.

**Why Each Wrong Option is Incorrect**
**Option A:** Digitalis is actually beneficial in HOCM as it increases contractility and heart rate, which can help to reduce the obstruction.
**Option B:** Atenolol is a beta blocker that is similar to propranolol and metoprolol, making it contraindicated in HOCM for the same reasons as the correct answer.
**Option C:** Calcium channel blockers, such as verapamil, are actually beneficial in HOCM as they can help to reduce the heart rate and contractility, which can help to reduce the obstruction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that beta blockers are generally contraindicated in HOCM, and calcium channel blockers may be beneficial in reducing the obstruction.

**Correct Answer: D. Calcium channel blockers, such as verapamil.**