**Core Concept**
D-dimer is a protein fragment produced when a blood clot dissolves. It is a sensitive marker for diagnosing conditions associated with fibrinolysis, such as thrombosis, disseminated intravascular coagulation (DIC), and pulmonary embolism.

**Why the Correct Answer is Right**
The correct answer is **C. Pulmonary Embolism**. D-dimer levels are elevated in patients with pulmonary embolism due to the formation of a blood clot in the lungs. The sensitivity of D-dimer for pulmonary embolism is high, but its specificity is low, meaning it can also be elevated in other conditions such as deep vein thrombosis, cancer, and inflammation. D-dimer levels are measured using a blood test and are often used as a screening tool to rule out pulmonary embolism in patients with a low pretest probability of the condition.

**Why Each Wrong Option is Incorrect**
**Option A:** D-dimer is not the most sensitive test for diagnosing deep vein thrombosis, although it can be elevated in this condition. Other tests such as ultrasound or venography are more specific for diagnosing deep vein thrombosis.

**Option B:** D-dimer is not the most sensitive test for diagnosing myocardial infarction. Troponin levels are more specific for diagnosing myocardial infarction.

**Clinical Pearl / High-Yield Fact**
The DVT-PE (Deep Vein Thrombosis-Pulmonary Embolism) Wells score is a clinical prediction rule that combines clinical and laboratory findings to estimate the probability of pulmonary embolism in patients. The score includes factors such as clinical symptoms, D-dimer levels, and imaging results.

**Correct Answer:** C. Pulmonary Embolism