Okay, let's tackle this question about the sodium content in WHO's oral rehydration solution. First, I need to remember the composition of ORS. The WHO standard ORS has specific concentrations for various electrolytes. Sodium is one of the key components because it helps with water absorption in the intestines.

I think the sodium concentration is around 75 mmol/L. Let me verify. The WHO ORS formula is designed for treating dehydration, especially in cases like diarrhea. The standard solution includes sodium chloride (NaCl), which provides sodium. The total sodium comes from NaCl, potassium chloride (KCl), and sometimes other sources. Wait, no, in the standard ORS, the main sodium source is NaCl. Let me recall the exact amounts. The WHO ORS formulation is 75 mmol/L sodium, 20 mmol/L potassium, 100 mmol/L glucose, and 10 mmol/L citrate. The sodium concentration is indeed 75 mmol/L. So the correct answer should be 75 mmol/L. Now, common incorrect options might be lower or higher values. For example, some might confuse it with the older formulation or other solutions like Ringer's lactate. The key is that the WHO ORS has 75 mmol/L sodium. The clinical pearl here is the exact composition, which is crucial for proper hydration without overloading electrolytes. So the correct answer is 75 mmol/L.

**Core Concept**
The World Health Organization (WHO) oral rehydration solution (ORS) is formulated to replace fluids and electrolytes lost in dehydration, particularly from diarrhea. Sodium concentration is critical for optimal water absorption via the sodium-glucose cotransport mechanism in the small intestine.

**Why the Correct Answer is Right**
WHO ORS contains **75 mmol/L sodium**, derived primarily from sodium chloride (NaCl). This concentration balances effective fluid absorption with safety, avoiding hypernatremia. The sodium-glucose cotransporter (SGLT1) in the jejunum uses the sodium gradient to absorb water and glucose, enhancing rehydration efficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** 65 mmol/L – Incorrect; this is lower than the WHO standard and would reduce rehydration efficacy.
**Option B:** 90 mmol/L – Incorrect; this exceeds the WHO ORS formula and risks hypernatremia.
**Option D:** 100 mmol/L – Incorrect; this matches Ringer’s lactate but not WHO ORS, which is hypotonic.

**Clinical Pearl / High-Yield Fact**
WHO ORS is **hypotonic** (245 mOsm/L) and contains **75 mmol/L sodium**, **20 mmol/L potassium**, **100 mmol/L glucose**, and **10 mmol/L citrate**. Remember "75-20-100" for sodium-potassium-glucose to avoid confusing it with other solutions like Hartmann’s or Ringer’s.

**Correct Answer: C. 75 mmol/L**

✓ Correct Answer: B. 75