Review Question - QID 212428

A 40-year-old woman presents to her primary care physician for a checkup. She has felt weaker lately and has lost some weight. She denies any urinary issues. Her BUN at her last visit 4 months ago was 45 mg/dL, and her creatinine was 2.0 mg/dL. Her laboratory studies this visit are seen below:

Hemoglobin: 8 g/dL
Hematocrit: 29%
Leukocyte count: 9,500/mm^3 with normal differential
Platelet count: 197,000/mm^3

Serum:
Na+: 139 mEq/L
Cl-: 100 mEq/L
K+: 5.9 mEq/L
HCO3-: 17 mEq/L
BUN: 59 mg/dL
Glucose: 99 mg/dL
Creatinine: 2.3 mg/dL
Ca2+: 9.0 mg/dL

Which of the following is the most likely diagnosis?