**Core Concept**
The management of myocardial infarction (MI) involves relieving pain, which is a crucial aspect of patient care. **Myocardial infarction** pain is caused by ischemia of the heart muscle, leading to the release of various chemical mediators. The goal is to reduce the workload of the heart and alleviate the pain.

**Why the Correct Answer is Right**
The most appropriate choice for relieving pain in MI is morphine, as it reduces the workload of the heart by decreasing **preload** and **afterload** through venodilation. Morphine also has an anxiolytic effect, which helps in reducing the **sympathetic tone**, thereby decreasing the oxygen demand of the heart.

**Why Each Wrong Option is Incorrect**
**Option A:** Other opioids may not be as effective in reducing the preload and afterload as morphine.
**Option B:** Nonsteroidal anti-inflammatory drugs (NSAIDs) are not the first choice for pain relief in MI due to potential adverse effects on kidney function and bleeding.
**Option D:** Beta-blockers, while useful in MI management, are not primarily used for pain relief.

**Clinical Pearl / High-Yield Fact**
Morphine is the **drug of choice** for pain relief in acute myocardial infarction due to its ability to reduce cardiac workload and provide anxiolysis.

**Correct Answer:** D. Morphine.