You are answering a USMLE-style medical question. Question: A 52-year-old man presents to the clinic with a headache and dizziness for the past month. The patient reports that about 4 weeks ago he developed a “nagging” headache. He thought he had a cold, so he tried over the counter ibuprofen that provided only mild and temporary relief. Recently, he also started feeling dizzy and nauseous, which only resolves when he steps outside to shovel the snow. The patient’s medical history is significant for hypertension, type II diabetes mellitus, and asthma. His medications include amlodipine, metformin, glimepiride, and a fluticasone/salmeterol inhaler. The patient has been living at his upstate cabin for the past 5 weeks but denies other recent travel. He denies smoking tobacco or illicit drug use. The patient’s temperature is 99°F (37.2°C), blood pressure is 130/78 mmHg, pulse is 72/min, and respirations are 16/min with an oxygen saturation of 98% on room air. The patient is currently asymptomatic while sitting in the office, and physical examination is unremarkable. Labs are drawn, as shown below: Hemoglobin: 20 g/dL Hematocrit: 60% Leukocyte count: 10,050 cells/mm^3 with normal differential Platelet count: 350,000/mm^3 Which of the following underlies the patient’s most likely diagnosis? Options: A) Medication side effect B) Myeloproliferative neoplasm C) Neuronal hyperexcitability D) Toxic exposure Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.