You are answering a USMLE-style medical question. Question: A 68-year-old man presents to his primary care physician with complaints of increased fatigue and back pain for the past year. His symptoms are progressively getting worse over the past few months despite adequate rest and trial of over the counter analgesics. His back pain is constant and non-radiating in nature. He denies any trauma to his back. His past medical history is insignificant. On examination, there is mild tenderness over the lumbar region in the midline with an absence of paraspinal muscle tenderness. Straight leg raise test is negative. His skin and conjunctiva appear pale. The physician orders basic blood work. His results are provided below. Complete blood count RBC 4.9 million cells/µL Hemoglobin 9.8 g/dL Hematocrit 41% Total leukocyte count 6,800 cells/µL Neutrophils 70% Lymphocyte 26% Monocytes 3% Eosinophil 1% Basophils 0% Platelets 230,000 cells/µL Basic Metabolic Panel Sodium 136 mEq/L Potassium 5.1 mEq/L Chloride 101 mEq/L Bicarbonate 24 mEq/L Albumin 3.6 mg/ dL Urea nitrogen 31 mg/dL Creatinine 2.7 mg/dL Uric Acid 6.7 mg/dL Calcium 12.1 mg/dL Glucose 105 mg/dL Urinalysis shows proteinuria without any RBCs or pus cells. The patient’s proteinuria best fits in which of the following category? Options: A) Overflow proteinuria B) Transient (functional) proteinuria C) Orthostatic proteinuria D) Isolated proteinuria Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.