Review Question - QID 108774

A 74-year-old woman with a past medical history of hypertension, peripheral artery disease, and migraine headaches presents to the emergency department with a 2 hour history of severe abdominal pain. The patient cannot recall any similar episodes, although she notes occasional abdominal discomfort after eating. She describes the pain as sharp periumbilical pain. She denies recent illness, fever, chills, nausea, vomiting, or diarrhea. Her last normal bowel movement was yesterday evening. Her temperature is 37.1°C (98.8°F), pulse is 110/min, blood pressure is 140/80 mmHg, and respirations are 20/min. On exam, the patient is grimacing and appears to be in significant discomfort. Heart and lung exams are within normal limits. The patient’s abdomen is soft and non-distended with diffuse periumbilical pain on palpation. There is no rebound tenderness or guarding, and bowel sounds are present. The rest of the exam is unremarkable. Labs in the emergency room show:

Serum:
Na+: 144 mEq/L
Cl-: 105 mEq/L
K+: 3.7 mEq/L
HCO3-: 20 mEq/L
BUN: 15 mg/dL
Glucose: 99 mg/dL
Creatinine: 1.2 mg/dL
Ca2+: 10.7 mg/dL
Phosphorus: 5.2 mg/dL
Lactate: 7.0 mmol/L
Amylase: 240 U/L

Hemoglobin: 13.4 g/dL
Hematocrit: 35%
Leukocyte count: 12,100 cells/mm^3 with normal differential
Platelet count: 405,000/mm^3

What is the most appropriate next step in diagnosis?