A client comes to the emergency department complaining of visual changes and a severe headache. The nurse measures the client's blood pressure at 210/120 mm Hg. However, the client denies having hypertension or any other disorder. After diagnosing malignant hypertension (hypertensive emergency), a life-threatening disorder, the physician initiates emergency intervention. What is the most common cause of malignant hypertension (hypertensive emergency)?
A) Pyelonephritis
B) Dissecting aortic aneurysm
C) Pheochromocytoma
D) Untreated hypertension