Final answer:
Hydralazine, a direct-acting vasodilator, should be avoided in acute aortic dissection as it increases shear stress within the aortic wall, potentially worsening the dissection.
Step-by-step explanation:
The antihypertensive medication that should be avoided in acute aortic dissection due to an increase in shear stress is vasodilators, particularly those that act directly, such as hydralazine. This type of medication can cause a reflexive increase in heart rate which, in turn, can lead to an increase in shear stress within the aortic wall. The increase in shear stress can exacerbate the dissection. Instead, the preferred initial treatment often involves a combination of a beta-blocker to reduce force of heart contractions and another agent to control blood pressure, such as intravenous nitroprusside, which reduces both preload and afterload without causing tachycardia.