What type of medications primarily act only on arteries and not veins, affecting afterload?

Dihydropyridine calcium channel blockers are the correct choice because these medications primarily work by selectively blocking calcium channels in vascular smooth muscle. This action leads to vasodilation, primarily affecting the arteries. By relaxing arterial smooth muscle, they decrease systemic vascular resistance and, consequently, reduce afterload on the heart.

This distinct mechanism is crucial in the management of hypertension, as it effectively lowers arterial pressure without significantly affecting venous pressure. In contrast, beta-blockers work by reducing heart rate and contractility rather than directly influencing the vascular system, while ACE inhibitors primarily act on the renin-angiotensin system to decrease blood pressure through various mechanisms, including effects on both arteries and veins. Thiazide diuretics mainly reduce blood volume by promoting urine production and have a more indirect effect on vascular resistance rather than specific arterial dilation.