Review Question - QID 213216

A 75-year-old man presents to the emergency department for abdominal pain. The patient states the pain started this morning and has been worsening steadily. He decided to come to the emergency department when he began vomiting. The patient has a past medical history of obesity, diabetes, alcohol abuse, and hypertension. His current medications include captopril, insulin, metformin, sodium docusate, and ibuprofen. His temperature is 104.0°F (40°C), blood pressure is 160/97 mmHg, pulse is 90/min, respirations are 15/min, and oxygen saturation is 98% on room air. Abdominal exam reveals left lower quadrant tenderness. Cardiac exam reveals a crescendo systolic murmur heard best by the right upper sternal border. Lab values are ordered and return as below.

Hemoglobin: 15 g/dL
Hematocrit: 42%
Leukocyte count: 19,500 cells/mm^3 with normal differential
Platelet count: 226,000/mm^3

Serum:
Na+: 139 mEq/L
Cl-: 101 mEq/L
K+: 4.4 mEq/L
HCO3-: 24 mEq/L
BUN: 22 mg/dL
Glucose: 144 mg/dL
Creatinine: 1.2 mg/dL
Ca2+: 9.8 mg/dL

Which of the following is the most accurate test for this patient's condition?