Review Question - QID 216419

A 27-year-old nurse presents to the emergency department with a 30-minute history of palpitations and chest pain. She has been experiencing nausea, vomiting, and diarrhea after eating leftovers 2 days ago. Her husband tells the physician that he is concerned that the patient has been increasingly obsessed about her weight recently. She has a history of major depressive disorder for which she takes escitalopram. Her temperature is 98.6°F (37.0°C), blood pressure is 110/72 mmHg, pulse is 110/min, and respirations are 11/min. Physical examination is notable for dry mucous membranes and tachycardia with regular rhythm. Laboratory testing shows the following results:

Serum:
Na+: 135 mEq/L
Cl-: 96 mEq/L
K+: 2.9 mEq/L
HCO3-: 36 mEq/L

Venous blood gas shows a pH of 7.52. Which of the following tests is most appropriate in differentiating between different etiologies of this patient's lab abnormalities?