Review Question - QID 215100

An 18-year-old man presents to his primary care physician for frequent loose stools over the course of 3 months and a pruritic rash. He reports that his stools are foul-smelling, oily, and often float in the toilet. He eats a fairly healthy diet, consisting of wheat and rye bread, nuts, lean meats, and vegetables. He lost approximately 15 pounds over the course of 2 months and has developed a rash involving both of his elbows and knees. He trialed a ketogenic diet which led to improvements in his bowel movements for 2 weeks; however, his symptoms resumed after he returned to his normal diet. He has no other significant past medical history and takes no medications. His temperature is 36.8°C (98.2°F), blood pressure is 115/75 mmHg, pulse is 68/min, respirations are 18/min, and oxygen saturation is 99% on room air. On physical examination, a rash is appreciated (Figure A). Which of the following diagnostic studies is the most appropriate next step in management?