Review Question - QID 217603

A 55-year-old woman presents to a primary care physician with persistent nausea and abdominal discomfort. She has experienced these symptoms daily for the past 3 months. She feels bloated and has episodic loose and watery stools. She has a history of hypertension and hyperlipidemia for which she takes amlodipine and atorvastatin. She immigrated from Vietnam 6 months ago. Her temperature is 98.6°F (37.0°C), blood pressure is 110/60 mmHg, pulse is 70/min, and respirations are 18/min. Physical exam reveals a thin-appearing woman in no acute distress. Her mucous membranes are moist. Cardiac and lung examinations are unremarkable. Laboratory results are as follows:

Leukocyte count: 13,000/mm^3
Segmented neutrophils: 54%
Bands: 2%
Eosinophils: 6%
Basophils: 0.4%
Lymphocytes: 30%
Monocytes: 5%

A vitamin D level is 26 ng/mL (reference: >= 30 ng/mL). Which of the following is the most appropriate test in making the diagnosis?