## Core Concept
Calcium channel blockers (CCBs) are a class of medications that inhibit the influx of calcium ions into cardiac and smooth muscle cells, leading to vasodilation and decreased myocardial contractility. They are primarily used to treat hypertension, angina pectoris, and certain types of arrhythmias. CCBs can be classified into two main types: dihydropyridines (e.g., nifedipine) and non-dihydropyridines (e.g., verapamil, diltiazem).

## Why the Correct Answer is Right
The correct answer, , states that calcium channel blockers decrease myocardial oxygen demand. This is accurate because CCBs achieve this by reducing systemic vascular resistance (afterload) through vasodilation, which in turn decreases the workload on the heart and its oxygen requirements. This mechanism is particularly beneficial in the management of angina pectoris.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because CCBs do not significantly increase myocardial oxygen supply directly; their primary benefit in angina is through reducing oxygen demand.
* **Option B:** This option is incorrect as CCBs can actually worsen heart failure in some patients, particularly those with significant left ventricular dysfunction, due to their negative inotropic effect.
* **Option C:** This option might seem plausible but is not the best answer given the context of the question focusing on a broadly true statement about CCBs.

## Clinical Pearl / High-Yield Fact
A key point to remember is that CCBs can have varying effects on the heart, depending on their type. For instance, dihydropyridine CCBs primarily cause vasodilation with minimal effect on heart rate or contractility, while non-dihydropyridines can significantly reduce heart rate and contractility.

## Correct Answer Line
**Correct Answer: D. Decrease myocardial oxygen demand.**