A 45-year-old man experiences crushing substernal chest pain after arriving at work one morning. Over the next 4 hours, the pain persists and begins to radiate to his left arm. He becomes diaphoretic and short of breath but waits until the end of his 8-hour shift to go to the hospital. An elevated serum value of which of the following laboratory tests would be most useful for the diagnosis of this patient on admission to the hospital?

**Core Concept**
The patient's presentation is consistent with acute myocardial infarction (AMI), also known as a heart attack. The underlying pathophysiological principle being tested is the role of cardiac biomarkers in diagnosing AMI. Cardiac biomarkers, such as troponins, are released into the bloodstream when cardiac muscle cells are damaged, indicating AMI.

**Why the Correct Answer is Right**
The elevated serum value of troponin I is the most useful laboratory test for diagnosing AMI. Troponin I is a cardiac-specific protein that is released into the bloodstream within 2-3 hours after cardiac muscle cell damage. Its levels continue to rise over the next 12-24 hours, making it a sensitive and specific marker for AMI. The patient's symptoms, such as crushing substernal chest pain and shortness of breath, are classic presentations of AMI, and an elevated troponin I level would confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Creatine kinase (CK) is a non-specific enzyme that can be elevated in various conditions, including muscle damage, making it less specific for diagnosing AMI.
**Option B:** Myoglobin is a non-specific marker that can be elevated in conditions such as rhabdomyolysis, making it less useful for diagnosing AMI.
**Option C:** Lactate dehydrogenase (LDH) is an enzyme that can be elevated in various conditions, including myocardial infarction, but it is not as specific or sensitive as troponin I for diagnosing AMI.

**Clinical Pearl / High-Yield Fact**
Remember that troponin I is a cardiac-specific biomarker that can be elevated within 2-3 hours after cardiac muscle cell damage, making it a sensitive and specific marker for acute myocardial infarction.

**Correct Answer:** C. Troponin I

✓ Correct Answer: C. CK-MB fraction