MRCP-0600

You are reviewing a 70-year-old gentleman who has been suffering from multiple myeloma for the past 3 years. He presents with lethargy, muscle aches and pain in his lower back. Arterial blood sampling reveals a metabolic acidosis. Serum potassium is 3.1 mmol/l (3.5–4.9), and urine pH is 5.1 (> 5.3). What is the most likely diagnosis?