**Core Concept:** Renal artery is the primary blood vessel supplying the kidneys with blood, and its diameter plays a crucial role in determining blood flow. In this question, we are considering the effect of a reduction in renal artery lumen on blood flow.

**Why the Correct Answer is Right:** When the diameter of a blood vessel, such as the renal artery, is reduced, the cross-sectional area decreases. According to the Poiseuille's Law, blood flow (Q) is directly proportional to the fourth power of the radius (r^4) and inversely proportional to the resistance (R), length (L), and viscosity (η) of the blood. In this case, the radius (r) is reduced, so the blood flow will decrease significantly.

**Why Each Wrong Option is Incorrect:**

A. **Not addressing the reduction in radius:** This option does not consider the primary factor affecting blood flow in this scenario, making it incorrect.
B. **Incorrect application of Poiseuille's Law:** This option might appear similar to the correct answer, but it incorrectly applies Poiseuille's Law, leading to an inaccurate result.
C. **Ignoring the effect of radius reduction:** Similar to option A, this option does not address the primary factor affecting blood flow and is therefore incorrect.
D. **Incorrect application of Poiseuille's Law:** This option incorrectly applies Poiseuille's Law, leading to an inaccurate result.

**Clinical Pearl:** An increase in resistance in the renal circulation due to arterial stenosis can lead to decreased glomerular filtration rate (GFR), which in turn can result in decreased urine production, hypertension, or even kidney damage, emphasizing the importance of promptly identifying and treating renal artery stenosis in clinical practice.

**Correct Answer:** Since the radius (r) in Poiseuille's Law is reduced, the correct answer is:

**Correct Answer: B. 40% decrease in blood flow**

Explanation:

The correct application of Poiseuille's Law, **Q = π * (8 * π * r^4 * ΔP / (8 * η * L)**, considers the reduction in radius (r) and applies it correctly. In this case, when the radius (r) is reduced by half, the blood flow (Q) will decrease by a factor of 4 (since r^4 is reduced by a factor of 1/4). Therefore, the blood flow will decrease by approximately 40%.