A 16-year-old girl presented to the emergency department with 1 week of severe, diffuse abdominal pain and constipation, as well as several episodes of non-bloody, non-bilious emesis.

A detailed history revealed that the patient had been previously healthy. Her symptoms began several days after she decreased her caloric intake in an attempt to lose weight. Additionally, she had been drinking 48 to 60 oz of water per day for several days before hospitalization, in an attempt to ameliorate her constipation. Furthermore, she admitted to having consumed alcohol the night before her pain began.

On examination, her abdomen was tender to palpation throughout, but soft with no rebound tenderness or peritoneal signs. No other relevant findings were detected clinically.

What investigations would you suggest for this patient?