A 38-year-old woman presents with a 6-month history of progressive headaches and palpitations. On examination, blood pressure is 168/104 mmHg bilaterally, heart rate 92/min regular, and there is a palpable abdominal mass in the left flank. Serum electrolytes show K⁺ 3.2 mEq/L (normal 3.5–5.0). Plasma metanephrines are elevated at 1.8 nmol/L (normal <0.9). 24-hour urine catecholamines are 850 µg/24 h (normal <100). CT abdomen shows a 4 cm left adrenal mass with heterogeneous enhancement. What is the most likely diagnosis?