**Core Concept:** Cardiac output is the product of heart rate and stroke volume. Kidneys require a certain amount of blood supply for filtration and waste removal. The percentage of cardiac output allocated to a particular organ is determined by the relative demand and supply balance in the body.

**Why the Correct Answer is Right:** Kidneys have a high demand for blood supply due to their role in maintaining homeostasis, particularly in the regulation of electrolytes, fluid balance, and blood pressure. The specific percentage of cardiac output allocated to the kidneys is approximately 20%. This value is derived from the fact that the heart pumps around 5-6 litres of blood per minute (heart rate × stroke volume), and the kidneys constitute around 0.5% of the total body weight, requiring 20% of the cardiac output to meet their needs.

**Why Each Wrong Option is Incorrect:**

A. Kidneys receive less than 20% of cardiac output, as it is the correct answer.
B. Kidneys receive more than 20% of cardiac output, which is incorrect as mentioned above.
C. Kidneys receive equal distribution of cardiac output, which is incorrect, as kidneys receive a lesser percentage due to their high demand.
D. Kidneys receive only 10% of cardiac output, which is incorrect, as the correct answer is 20%.

**Clinical Pearl:** The heart rate × stroke volume × 20% (kidneys' percentage of cardiac output) calculation can be used to estimate the blood flow to any organ in relation to the cardiac output. This is useful for clinical scenarios where direct measurement is not possible, such as estimating blood flow to a limb or organ.

**Correct Answer:** Kidneys receive approximately 20% of cardiac output (0.02 × 50 = 1L/min).