Review Question - QID 214479

A 50-year-old woman presents to the clinic with progressive back pain. She states she fell 2 years ago, and her back has bothered her since. She thinks the pain affects her appetite. She has lost 5 pounds over the past 4 months. On physical examination, the patient has pallor, midline tenderness upon palpation of L4 and L5, and 1+ pitting edema of both lower extremities to the knees. Laboratory investigation reveals the following results:

Serum:
Na+: 137 mEq/L
Cl-: 105 mEq/L
K+: 4.3 mEq/L
HCO3-: 27 mEq/L
BUN: 20 mg/dL
Glucose: 100 mg/dL
Creatinine: 1.3 mg/dL
Total protein: 7 mg/dL
Albumin: 2 mg/dL
Leukocyte count: 11,000/mm^3
Hemoglobin: 8.0 g/dL
Platelets: 200,000/mm^3

A urine dipstick is positive for 3+ albumin. A radiograph of the spine shows a lytic bone lesion spanning from L4 to S1. Which of the following is the most likely diagnosis?