Mallory-Weiss syndrome

A 28-year-old man is brought to the emergency department for evaluation of upper gastrointestinal bleeding. The patient had spent the evening at a bar with friends and consumed 7-10 shots of liquor. Shortly after leaving the bar, he felt nauseated and vomited multiple times. The last episode of vomiting produced bright red emesis. The patient denies dyspnea or recent trauma. Past medical history is notable for asthma, for which he takes albuterol as needed. His vitals are within normal limits. Physical examination reveals a thin male in mild distress. Cardiac, pulmonary, and abdominal exams are noncontributory. The patient has no additional episodes of hematemesis. Laboratory testing reveals the following findings:  
 

Laboratory value	Result
Sodium	139 mg/dL
Potassium	3.8 mg/dL
Chloride	100 mg/dL
Bicarbonate	33 mg/dL

Which of the following would be most useful in providing a definitive diagnosis of the patient’s condition?