Review Question - QID 108588

A 43-year-old woman presents to her physician’s office complaining of fatigue and lightheadedness for 1 month. She says she does not recall any triggers that may have started the symptoms. Her past medical history is significant for diabetes for which she takes metformin. She has regular periods but notes that they have become heavier in the last year. She endorses increased urination and feels that she has gained weight in her abdomen. A review of systems is otherwise negative. She is a daycare teacher and has a first cousin with von Willebrand disease. Her temperature is 98.4°F (36.9°C), pulse is 92/min, blood pressure is 109/72 mmHg, and respirations are 14/min.

A CBC demonstrates:
Hemoglobin: 9.9 g/dL
Leukocyte count: 6,300/mm^3
Platelet count: 180,000/mm^3

Which of the following is the best next step to evaluate the etiology of this patient’s findings?