MRCP2-2246
A 72-year-old man presents with fatigue and pain in his lower back and left femur. He is currently undergoing investigations for anemia and is scheduled for an upper GI endoscopy. Upon examination, the following results were obtained: hemoglobin (Hb) level of 89 g/l (normal range: 135-175 g/l), viscosity of 2.5 (normal range: 1.5-1.72), creatinine (Cr) level of 210 μmol/l (normal range: 50-120 mol/l), negative fecal occult blood test, and a prostate-specific antigen (PSA) level of 4. X-ray of the left femur revealed multiple osteolytic lesions. What is the most likely underlying diagnosis?