**Core Concept**
ACE (Angiotensin-Converting Enzyme) inhibitors are a class of medications used to treat hypertension, heart failure, and diabetic nephropathy. They work by blocking the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor, thereby reducing peripheral resistance and blood pressure.

**Why the Correct Answer is Right**
The most significant effect of ACE inhibition is the reduction of afterload, which is the resistance against which the heart must pump blood. This is achieved by decreasing the levels of angiotensin II, a powerful vasoconstrictor that increases peripheral resistance. As a result, ACE inhibitors cause vasodilation, particularly in the arterioles, leading to a decrease in blood pressure. Additionally, ACE inhibitors also have a protective effect on the kidneys by reducing glomerular pressure and proteinuria.

**Why Each Wrong Option is Incorrect**
**Option A:** While ACE inhibitors do have a beneficial effect on cardiac output, this is not their most significant effect.
**Option B:** Although ACE inhibitors can reduce aldosterone levels, this is not the primary mechanism by which they exert their therapeutic effects.
**Option C:** ACE inhibitors do not primarily act by increasing the levels of bradykinin, although this may be a secondary effect.

**Clinical Pearl / High-Yield Fact**
Remember that ACE inhibitors can cause a first-dose hypotension, particularly in patients with heart failure or those taking diuretics. This is due to the sudden reduction in afterload and the resulting decrease in blood pressure.

**Correct Answer: C. Reduction of afterload.**