Review Question - QID 103009

A 50-year-old man presents to the emergency department with abdominal pain. He has had abdominal pain associated with meals for the past several months for which he had been taking over-the-counter antacids as needed. However, he began experiencing significant worsening of the abdominal pain 1 hour ago in the epigastric region radiating to the right shoulder with nausea and vomiting. He denies fever, chills, or cough. He has a history of hypertension for which he takes amlodipine. He has not had any prior surgeries. He works as a carpenter and does not smoke cigarettes, drink alcohol, or use illicit drugs. The patient's temperature is 101.0°F (38.3°C), blood pressure is 110/60 mmHg, pulse is 110/min, and respirations are 22/min. Physical exam reveals diffuse abdominal rigidity with rebound tenderness. Auscultation of his abdomen reveals hypoactive bowel sounds. Which of the following is the best next step in the diagnosis of this patient's condition?