A 42-Year-Old Woman with Elevated Blood Pressure, Visual Disturbances, and Vomiting

A 42-year-old woman with elevated blood pressure, visual disturbances, and vomiting has increased urinary excretion of 3-methoxy-4-hydroxymandelic acid (VMA) . A computerized tomographic scan shows an adrenal mass that is consistent with a diagnosis of pheochromocytoma. While awaiting surgery to remove the tumor, she is treated with phenoxybenzamine to lower her blood pressure. What is the mechanism of this action of the drug?

A) Increasing cyclic adenosine monophosphate (cAMP)
B) Decreasing cAMP
C) Increasing inositol 1,4,5-triphosphate $\left(\mathrm{IP}_{3}\right) / \mathrm{Ca}^{2+}$
D) Decreasing $\mathrm{IP}_{3} / \mathrm{Ca}^{2+}$
E) Opening $\mathrm{Na}^{+} / \mathrm{K}^{+}$ channels
F) Closing $\mathrm{Na}^{+} / \mathrm{K}^{+}$ channels