**Core Concept:** Osmolarity is the number of particles in a solution, which affects the concentration of solutes and the osmotic pressure. In oral rehydration therapy (ORT), a solution with low osmolarity is used to prevent water absorption and maintain electrolyte balance in cases of diarrhea. The primary electrolytes in ORS are sodium, potassium, chloride, and bicarbonate.

**Why the Correct Answer is Right:** A low osmolar oral rehydration solution (ORS) contains a lower concentration of sodium to prevent excessive water absorption and maintain electrolyte balance. The World Health Organization (WHO) recommends an ORS with a sodium concentration of 70 mmol/L (moles of solute per liter) for better efficacy in treating diarrheal diseases.

**Why Each Wrong Option is Incorrect:**

A. **Incorrect:** An excessively high sodium concentration would lead to excessive water absorption, worsening dehydration instead of improving it.
B. **Incorrect:** Sodium concentration should ideally be lower than 100 mmol/L to prevent severe complications.
C. **Incorrect:** The correct sodium concentration is closer to 70 mmol/L, as mentioned by the WHO guidelines.
D. **Incorrect:** A sodium concentration greater than 120 mmol/L could lead to complications and is not recommended.

**Clinical Pearl:** A low osmolar ORS with sodium concentration around 70 mmol/L is crucial for effective rehydration therapy in cases of diarrhea. This is essential for maintaining electrolyte balance and preventing severe complications.

**Correct Answer:** C. 70 mmol/L (moles of solute per liter)