Hematologic Malignancies USMLE Step 1 Practice Question

A 67-year-old man presents to the emergency department with progressive fatigue, bone pain, and recurrent infections over 3 months. His wife notes he has become increasingly confused over the past week. Vital signs: BP 148/92 mmHg, HR 88/min, RR 16/min, Temp 37.2°C. Laboratory studies show: hemoglobin 8.5 g/dL, creatinine 2.8 mg/dL, corrected calcium 11.5 mg/dL, albumin 2.8 g/dL. Serum protein electrophoresis demonstrates a monoclonal spike, and immunofixation identifies IgG lambda. Bone marrow biopsy shows 35% clonal plasma cells with kappa/lambda ratio of 100:1. Urinalysis is negative for proteinuria. Which of the following is the most likely diagnosis?