**Core Concept**
Myocardial ischemia occurs when blood flow to the heart muscle is reduced, leading to a lack of oxygen and nutrients. Detecting reversible ischemia is crucial for managing patients at risk of heart disease. The test in question assesses the heart's functional capacity and perfusion.

**Why the Correct Answer is Right**
The test of choice for detecting reversible myocardial ischemia is the **Stress Echocardiogram (SE)**. During SE, the patient undergoes physical stress (exercise or pharmacological stress) while an echocardiogram is performed. This allows for the assessment of wall motion abnormalities and perfusion defects, indicating areas of reversible ischemia. The **nitroglycerin stress test** is also an option, but it's less commonly used due to its lower sensitivity.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Cardiac MRI** is a highly sensitive imaging modality for detecting myocardial infarction and fibrosis but is not the primary test for reversible ischemia.
* **Option B:** **Holter Monitoring** records the heart's electrical activity over a 24-hour period and is used to diagnose arrhythmias and monitor pacemaker function, not reversible ischemia.
* **Option C:** **Electrocardiogram (ECG)** is a basic test for detecting acute myocardial infarction but is not suitable for detecting reversible ischemia.

**Clinical Pearl / High-Yield Fact**
In patients with known or suspected coronary artery disease, a stress echocardiogram is a valuable tool for detecting reversible ischemia and guiding treatment decisions.

**Correct Answer:** C.