Which diagnostic test uses radioactive potassium to assess heart muscle?

The Thallium-201 stress test is the correct answer as it specifically utilizes radioactive isotopes, in this case, thallium-201, to assess blood flow to the heart muscle during stress conditions. In this diagnostic test, thallium is injected into the bloodstream, and its distribution in the heart is observed using a gamma camera after the heart has been put under stress, often through exercise or pharmacological means. This technique allows healthcare providers to determine areas of the heart that may not be getting sufficient blood flow, indicating potential ischemia or damage.

The other options represent different types of diagnostic tests. An electrocardiogram captures the electrical activity and rhythm of the heart but does not utilize radioactive materials. An echocardiogram uses ultrasound waves to create images of the heart's structures and function, also not involving radioactivity. Cardiac MRI provides detailed images of the heart’s anatomy and function using magnetic resonance technology, which does not entail the use of radioactive substances. Thus, the unique use of radioactive thallium in the stress test clearly identifies it as the proper answer for assessing heart muscle health.