**Core Concept**
Peripheral resistance refers to the opposition to blood flow in the peripheral circulation, primarily determined by the resistance in the small arteries, arterioles, and capillaries. It plays a crucial role in regulating blood pressure and maintaining tissue perfusion.

**Why the Correct Answer is Right**
Diastolic blood pressure is the best indicator of peripheral resistance because it reflects the pressure in the arteries during diastole, when the heart is not pumping blood. Peripheral resistance is the primary determinant of diastolic blood pressure, as it opposes the flow of blood through the small arteries and arterioles. The diastolic pressure is a reflection of the resistance offered by the peripheral circulation, making it the most accurate indicator of peripheral resistance.

**Why Each Wrong Option is Incorrect**
**Option B:** Pulse pressure is the difference between systolic and diastolic blood pressure, which is not a direct indicator of peripheral resistance. While it can be affected by peripheral resistance, it is also influenced by other factors such as cardiac output and arterial compliance.

**Option C:** Systolic resistance in the aorta increases with its length, but this is not a direct indicator of peripheral resistance. The aorta is a large elastic artery, and its resistance is not representative of the peripheral circulation.

**Option D:** Mean arterial pressure is responsible for blood flow to an organ, but it is not the best indicator of peripheral resistance. Mean arterial pressure is influenced by both cardiac output and peripheral resistance, making it a less accurate indicator of peripheral resistance.

**Clinical Pearl / High-Yield Fact**
Peripheral resistance is directly influenced by the tone of the small arteries and arterioles, which is regulated by the sympathetic nervous system. An increase in sympathetic tone leads to vasoconstriction and an increase in peripheral resistance, while a decrease in sympathetic tone leads to vasodilation and a decrease in peripheral resistance.

**✓ Correct Answer: A. Diastolic blood pressure**