A 52-year-old man attends with fatigue and generalised aches. Hb is 9.8 g/dL, MCV 88 fL. Serum ferritin is normal, but serum calcium is 2.80 mmol/L and creatinine is mildly elevated. ESR is markedly raised. Urinalysis shows proteinuria. What is the most important investigation to confirm the suspected diagnosis?