A 23-year-old woman with no other medical history was diagnosed with hypertension 6 months ago. She was initially treated with hydrochlorothiazide, followed by the addition of lisinopril, followed by a calcium channel blocker, but her blood pressure has not been well controlled. She assures the provider that she is taking all of her medicines. On examination her blood pressure is 165/105 in each arm, and 168/105 when checked by large cuff in the lower extremities. Her pulse is 60. Cardiac examination reveals an S4 gallop but no murmurs. She has a soft mid-abdominal bruit. Distal pulses are intact and equal. She does not have hyperpigmentation, hirsutism, genital abnormalities, or unusual distribution of fat. Her sodium is 140, potassium 4.0, HCO3 22, BUN 15, and creatinine 1.5. Which of the following is the most likely cause of her difficult-to-control hypertension?