A patient in their 50s with a history of hypertension reported having 30 minutes of persistent chest pain. All vital signs were normal except for an elevated blood pressure of 188/104 mm Hg. Laboratory results (hemogram; serum electrolytes; kidney and hepatic; and levels of troponin I and D-dimer) were all within normal limits. An electrocardiogram (ECG) on admission is shown in the Figure, A. The patient was diagnosed with acute myocardial infarction (AMI) and prescribed 300 mg of aspirin and 600 mg of clopidogrel. The emergency coronary angiography findings were normal. A repeated ECG (Figure, B) after coronary angiography showed sinus rhythm without obvious ST-T deviation. Although the ECG findings had shown a significant improvement, the patient’s chest pain still persisted. Over the next 8 hours, serial ECGs remained unchanged, and the troponin I level remained unremarkable. However, D-dimer levels significantly increased to 10.2 μg/mL (reference value, <0.25 μg/mL; to convert to nmol/L, multiply by 5.476).