**Core Concept**
Reversible myocardial ischemia refers to a condition where the heart muscle does not receive enough blood flow, leading to temporary damage. This condition can be detected through various tests that assess myocardial perfusion.

**Why the Correct Answer is Right**
The test used to detect reversible myocardial ischemia is the **Thallium-201 myocardial perfusion scan** or **Thallium stress test**. This test involves the injection of a radioactive tracer, Thallium-201, which is taken up by the myocardium in proportion to blood flow. During exercise or pharmacological stress, areas of the heart with reduced blood flow will have decreased Thallium uptake, indicating reversible ischemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **Exercise Echocardiography** is used to assess wall motion abnormalities and left ventricular function during exercise, but it does not specifically detect reversible ischemia.
**Option B:** This option is incorrect because **Cardiac MRI** is a useful tool for evaluating cardiac structure and function, but it is not the primary test for detecting reversible ischemia.
**Option C:** This option is incorrect because **Echocardiography** is a valuable tool for assessing cardiac function and structure, but it is not specific for detecting reversible ischemia.

**Clinical Pearl / High-Yield Fact**
The Thallium stress test is particularly useful in patients with chest pain or shortness of breath who have a normal or near-normal electrocardiogram (ECG) and cardiac enzymes.

**Correct Answer:** D. Thallium-201 myocardial perfusion scan.