Review Question - QID 214934

A 42-year-old woman presents to the emergency room reporting a 1-month history of rash, dyspnea, joint pain, and tingling sensations in her bilateral upper and lower extremities. She has a history of hypertension which is treated with lifestyle interventions, but she otherwise has no documented medical history. She does not take any medications. She drinks 2 glasses of wine a day, has smoked a pack of cigarettes daily for the past 5 years, and has previously used intravenous drugs. She reports she has not used intravenous drugs for the past 2 years. Her temperature is 99.5°F (37.7°C), blood pressure is 155/90 mmHg, pulse is 100/min, and respirations are 16/min. Skin examination reveals palpable purpura and skin ulcers on both upper extremities. Her physical exam is otherwise notable for decreased ankle jerk reflexes. Laboratory results are as follows:

Hemoglobin: 12 g/dL
Platelet count: 240,000 / mm^3
Creatinine: 2.5 mg/dL
Blood urea nitrogen: 35 mg/dL
Total C4 complement level: Low
Rheumatoid Factor: Positive
Aspartate aminotransferase (AST): 120 U/L
Alanine aminotransferase (ALT): 168 U/L
Human immunodeficiency virus (HIV)-1 antibody: Negative

Which of the following would be most helpful in establishing this patient’s diagnosis?