You are answering a USMLE-style medical question. Question: 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? Options: A) Acute kidney failure B) Chronic kidney failure C) Obstructive uropathy D) Renal cell carcinoma Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.