**Core Concept**
In the management of dehydration due to diarrhea in children, Oral Rehydration Solutions (ORS) play a crucial role in maintaining fluid balance. The World Health Organization (WHO) recommends a specific amount of ORS based on the child's weight to effectively rehydrate the body.

**Why the Correct Answer is Right**
The correct amount of ORS for a 4 kg child is 75 mL. This is based on the WHO's recommendation of 75 mL/kg of ORS in the first 4 hours, given in divided doses over 4-6 hours. The ORS solution contains a balanced mixture of electrolytes, glucose, and water that helps to replace lost fluids and electrolytes due to diarrhea. The recommended amount ensures that the child receives adequate fluids to prevent dehydration and its complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may be a guess, but it does not follow the WHO's recommended guidelines for ORS administration in children with diarrhea.
**Option B:** This option may be a common mistake, but the correct amount of ORS for a 4 kg child is 75 mL, not 50 mL.
**Option C:** This option is incorrect because the recommended amount of ORS is based on the child's weight, not their age.

**Clinical Pearl / High-Yield Fact**
In children with diarrhea, it's essential to monitor their urine output and stool frequency to assess the effectiveness of ORS administration. A child who is well-hydrated will have a good urine output and a decrease in stool frequency.

**Correct Answer: D. 75 mL**