Image of the Week: What is causing this patient's resistant hypertension?

A 54-year-old man presents with fatigue, lethargy, and frequent headaches. He has a 1-year history of hypertension that has been resistant to treatment, and on examination his blood pressure is 145/82 mm Hg. Laboratory tests reveal a serum potassium level of 3.1 mmol/L. Subsequent examination demonstrates an increased ratio of plasma aldosterone concentration to plasma renin activity and the findings ...