**Core Concept:** Oral Rehydration Solution (ORS) is a solution used to treat dehydration caused by diarrhea. It is a safe, cost-effective, and widely available method of rehydration therapy.

**Why the Correct Answer is Right:** In the case of a 20 kg child, the correct answer is **D.** The amount of ORS required during the first 4 hours is calculated based on the child's weight and fluid losses due to diarrhea. The formula for calculating ORS is:

ORS (in milliliters) = 100 ml/kg body weight x (number of diarrheal stools per day) + 20 ml/kg body weight

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it assumes that the ORS needs to be given at a fixed rate of 50 ml/kg (8 glasses) per day, which is an outdated practice and does not consider the child's body weight.

B. This option is incorrect because it does not consider the number of diarrheal stools per day, which is a significant factor in determining the fluid loss and subsequent ORS requirement.

C. This option is incorrect as it does not calculate the fluid loss due to diarrhea, which is crucial for determining the required ORS volume.

**Clinical Pearl:** A clinical pearl to remember is that the volume of ORS required should be adjusted based on the child's weight and number of diarrheal stools per day. This ensures that the child receives an appropriate amount of fluids to compensate for the fluid loss due to diarrhea.

**Correct Answer:** D. ORS (in milliliters) = 100 ml/kg body weight x (number of diarrheal stools per day) + 20 ml/kg body weight