**Core Concept**
The key to differentiating between stable angina and NSTEMI lies in the level of myocardial damage and the presence of troponin elevation. Stable angina is characterized by transient myocardial ischemia without significant myocardial damage, whereas NSTEMI involves myocardial necrosis and elevated troponin levels.

**Why the Correct Answer is Right**
The correct answer is based on the pathophysiological distinction between stable angina and NSTEMI. In stable angina, transient myocardial ischemia occurs due to coronary artery stenosis, without significant myocardial damage. In contrast, NSTEMI involves plaque rupture, leading to thrombus formation and subsequent myocardial necrosis, resulting in elevated troponin levels. This distinction is crucial for determining the appropriate management strategy.

**Why Each Wrong Option is Incorrect**
**Option A:** Elevated troponin levels are not specific to NSTEMI and can be seen in other conditions such as renal failure, heart failure, and acute coronary syndromes.
**Option B:** Myocardial biopsy is an invasive procedure and is not a practical or widely used method for differentiating between stable angina and NSTEMI.
**Option C:** ECG changes, such as ST-segment depression, can be seen in both stable angina and NSTEMI, and are not specific enough to differentiate between the two conditions.

**Clinical Pearl / High-Yield Fact**
Remember that troponin elevation is a sensitive marker for myocardial necrosis and is a key differentiator between stable angina and NSTEMI.

**Correct Answer:** C. Myocardial biopsy.