**Core Concept**
Cardiotoxicity caused by radiotherapy and chemotherapy is a serious side effect that can lead to cardiac dysfunction and even heart failure. This condition is often associated with anthracycline chemotherapy agents, such as doxorubicin, and radiation therapy to the chest. The cardiotoxic effects of these treatments can be detected through various diagnostic tests.

**Why the Correct Answer is Right**
The best diagnostic test for detecting cardiotoxicity caused by radiotherapy and chemotherapy is the **Echocardiogram**. This non-invasive test uses sound waves to create images of the heart, allowing clinicians to assess cardiac function and detect any changes in heart structure and function. Specifically, echocardiography can measure parameters such as left ventricular ejection fraction (LVEF), wall motion abnormalities, and diastolic dysfunction. These measurements can help identify early signs of cardiotoxicity, enabling timely intervention and potentially preventing further cardiac damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Troponin levels are elevated in cardiac injury, but they are not specific for cardiotoxicity caused by radiotherapy and chemotherapy.
**Option B:** Electrocardiogram (ECG) changes can occur in cardiotoxicity, but they are non-specific and not sensitive enough to detect early changes.
**Option C:** Cardiac magnetic resonance imaging (MRI) is a sensitive test for detecting cardiotoxicity, but it is not the first-line diagnostic test due to its cost and limited availability.

**Clinical Pearl / High-Yield Fact**
When evaluating patients for cardiotoxicity, it is essential to assess cardiac function before starting treatment with anthracyclines or radiation therapy to the chest. Early detection of cardiotoxicity can significantly improve patient outcomes.

**Correct Answer: C. Cardiac magnetic resonance imaging (MRI) is a sensitive test for detecting cardiotoxicity, but it is not the first-line diagnostic test due to its cost and limited availability.