## **Core Concept**
The reduced osmolarity Oral Rehydration Solution (ORS) is a type of fluid replacement used to treat dehydration caused by diarrhea. It contains essential salts and glucose to help the body absorb fluids. The composition of ORS is critical for its effectiveness.

## **Why the Correct Answer is Right**
The reduced osmolarity ORS formula, recommended by the World Health Organization (WHO), contains sodium (Na+), potassium (K+), chloride (Cl-), citrate, and glucose. The reduced osmolarity ORS has a lower concentration of sodium and glucose compared to the original formula. **Potassium** is an essential component, helping to replenish lost potassium ions.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the ion it represents is actually a component of the ORS.
- **Option B:** This option is incorrect because this ion is also a part of the ORS.
- **Option C:** This option is incorrect because this is a key component, helping with fluid absorption.

## **Why Option D is Correct**
- **Option D:** This option suggests that the ion not contained is **Lactate**. The reduced osmolarity ORS actually contains **citrate**, not lactate, as the base.

## **Clinical Pearl / High-Yield Fact**
A critical point to remember is that the reduced osmolarity ORS has been shown to be as effective as, or even more effective than, the original formula in treating dehydration due to diarrhea, with fewer side effects. The composition includes 75 mEq/L of sodium, 20 mEq/L of potassium, 65 mEq/L of chloride, 10 mEq/L of citrate, and 75 mmol/L of glucose.

## **Correct Answer:** D. Lactate.