Multiple myeloma

A 65 year-old man comes to his primary care PA because of lower back pain, recurrent sinus infections, and 4 kg (8.8 lb) unintentional weight loss over the past three months. Past medical history is notable for osteoporosis and atherosclerosis. The patient consumes 4-5 glasses of wine per week and is sexually active with his spouse. His temperature is 36.8°C (98.2°F), blood pressure is 125/76 mmHg, pulse is 65/min, and respiratory rate is 15/min. Cardiac, pulmonary, and abdominal exams are normal. Pain is elicited with palpation of the lumbar spine. As part of the diagnostic work-up, a peripheral blood smear is ordered and reveals the stacking of red blood cells (Rouleaux formation), as depicted here:  


  Image from: Wikimedia

Which of the following findings will provide a definitive diagnosis of the patient’s condition?