## Core Concept
The reduced osmolarity Oral Rehydration Solution (ORS) formula, recommended by the World Health Organization (WHO), is designed to treat dehydration caused by diarrhea. The composition of ORS is crucial for effective rehydration and electrolyte replenishment. Potassium is an essential component, helping to correct the electrolyte imbalance.

## Why the Correct Answer is Right
The correct composition of reduced osmolarity ORS as per WHO includes 75 mmol/litre of sodium and 20 mmol/litre of potassium, among other components. The potassium content of 20 mmol/litre is specifically chosen to help replenish the potassium lost through diarrhea, which is crucial for maintaining proper muscle and nerve function, and for the heart.

## Why Each Wrong Option is Incorrect
- **Option A:** 10 mmol/litre - This is lower than the recommended amount and might not adequately replenish potassium losses.
- **Option B:** 15 mmol/litre - Still lower than the recommended 20 mmol/litre, potentially insufficient for optimal potassium replenishment.
- **Option D:** 30 mmol/litre - This is higher than the recommended amount and could potentially lead to hyperkalemia.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the reduced osmolarity ORS has a specific composition that has been shown to be more effective and safer than the original formula. The reduced osmolarity solution has 75 mmol/litre of sodium and 20 mmol/litre of potassium, making it a critical tool in the management of dehydration, especially in pediatric patients.

## Correct Answer: C. 20.