Review Question - QID 216626

A 55-year-old man presents to the emergency department after feeling lightheaded then fainting while he was carrying boxes upstairs. The patient works on a farm and does not see a doctor often. He has a history of hypertension but does not take medications routinely. He has not had any chest pain, dyspnea, or weakness during this time frame and currently feels at his baseline. He smokes cigarettes occasionally. His temperature is 98.1°F (36.7°C), blood pressure is 152/93 mmHg, pulse is 86/min, respirations are 15/min, and oxygen saturation is 98% on room air. Physical exam is unremarkable and the patient walks with a stable gait and no focal weakness. An ECG is performed as seen in Figure A. Laboratory studies are ordered as seen below.

Hemoglobin: 12 g/dL
Hematocrit: 36%
Leukocyte count: 6,500/mm^3 with normal differential
Platelet count: 197,000/mm^3

Serum:
Na+: 140 mEq/L
Cl-: 102 mEq/L
K+: 4.3 mEq/L
HCO3-: 24 mEq/L
BUN: 20 mg/dL
Glucose: 139 mg/dL
Creatinine: 1.1 mg/dL
Troponin: < 0.01 ng/mL

Which of the following is the most likely diagnosis?