## Core Concept
The question pertains to a drug that exemplifies pharmacological success but therapeutic failure due to the coronary steal phenomenon. This phenomenon occurs when a drug causes vasodilation of normal coronary arteries but not in stenotic segments, potentially worsening ischemia in areas supplied by stenotic arteries.

## Why the Correct Answer is Right
The correct answer, **Diltiazem**, is a calcium channel blocker that primarily acts by dilating coronary arteries and reducing myocardial oxygen demand. However, it can cause the coronary steal phenomenon, where there's a preferential dilation of normal coronary vessels over stenotic ones, potentially exacerbating ischemia in areas supplied by stenotic arteries. This mechanism explains both its pharmacological success (effective vasodilation) and therapeutic failure (worsening of angina due to steal phenomenon).

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although nitrates can cause vasodilation, they are not typically associated with the coronary steal phenomenon in the context provided.
- **Option B:** This option is incorrect as beta-blockers decrease myocardial oxygen demand by reducing heart rate and contractility but are not commonly linked with the coronary steal phenomenon.
- **Option C:** This option is incorrect because, while dipyridamole can cause coronary steal phenomenon, it is not as classically associated with this therapeutic failure context as another option.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the coronary steal phenomenon is a critical consideration in the management of angina, particularly when using vasodilators. **Diltiazem**, a calcium channel blocker, is a notable example where pharmacological efficacy (vasodilation) does not translate to therapeutic success (relief of angina) due to this phenomenon.

## Correct Answer: D. Diltiazem