Osmolarity of ORS fluid –
**Core Concept**
The correct osmolarity of Oral Rehydration Solution (ORS) fluid is a critical parameter in managing dehydration, especially in cases of diarrhea or vomiting. The ideal osmolarity helps in preventing excessive water absorption, which can lead to hyponatremia.
**Why the Correct Answer is Right**
The World Health Organization (WHO) recommends an osmolarity of 245 mOsm/L for ORS, which is achieved by using a specific combination of sodium chloride, sodium citrate, glucose, and potassium chloride. This concentration is close to the osmolarity of human plasma, allowing for efficient rehydration without causing an osmotic imbalance. The glucose in ORS helps to facilitate sodium and water absorption in the intestine through cotransport mechanisms.
**Why Each Wrong Option is Incorrect**
**Option A:** An osmolarity of 100 mOsm/L would be too low, potentially causing excessive water absorption and leading to hyponatremia.
**Option B:** An osmolarity of 500 mOsm/L would be too high, potentially causing an osmotic imbalance and worsening dehydration.
**Option C:** An osmolarity of 300 mOsm/L is close but not the recommended concentration, which could lead to suboptimal rehydration.
**Clinical Pearl / High-Yield Fact**
Remember the "WHO ORS formula": 2.6 grams of sodium chloride, 2.9 grams of glucose, and 1.5 grams of potassium chloride per liter of water, aiming for an osmolarity of 245 mOsm/L.
**Correct Answer: D. 245**