**Core Concept**
Troponin is a cardiac-specific biomarker used for the diagnosis of myocardial infarction (MI). It is elevated in the blood within 2-3 hours after the onset of MI and remains elevated for up to 10-14 days.

**Why the Correct Answer is Right**
Troponin levels peak at 2-3 days and remain elevated for up to 10-14 days after the onset of MI. This prolonged elevation makes troponin an ideal marker for diagnosing MI even on the 10th day after the event. The high sensitivity and specificity of troponin for cardiac damage make it a reliable marker for ruling in or ruling out MI.

**Why Each Wrong Option is Incorrect**
**Option A:** CK-MB is another cardiac biomarker, but its levels return to normal within 48-72 hours after MI, making it less useful for diagnosing MI on the 10th day.

**Option B:** Myoglobin levels are elevated in MI, but they are not specific to the heart and can be elevated in other conditions, such as skeletal muscle damage.

**Option C:** LDH is a nonspecific marker of tissue damage and can be elevated in various conditions, including MI, but its levels do not accurately reflect the extent of cardiac damage.

**Clinical Pearl / High-Yield Fact**
Remember that troponin is a cardiac-specific biomarker, and its elevation is diagnostic of MI. However, it is essential to consider the clinical context and other diagnostic findings to confirm the diagnosis.

**Correct Answer:** A.