## Core Concept
The maintenance fluid requirement in children is calculated based on their weight. The **Holliday-Segar formula** is commonly used for this purpose, which estimates the daily fluid requirements as 100 mL/kg/day for the first 10 kg of body weight, 50 mL/kg/day for the next 10 kg (from 11-20 kg), and 20 mL/kg/day for weight over 20 kg.

## Why the Correct Answer is Right
For a child weighing 10 kg, the maintenance fluid requirement according to the Holliday-Segar formula is **100 mL/kg/day**. Therefore, for a 10 kg child, the daily maintenance fluid requirement would be 100 mL/kg/day * 10 kg = **1000 mL/day**. To find the hourly rate, we divide this by 24 hours: 1000 mL/day / 24 hours/day = approximately **42 mL/hour**.

## Why Each Wrong Option is Incorrect
- **Option A:** Without a specific value, it's hard to directly refute, but any option not matching 42 mL/hour for a 10 kg child would be incorrect based on the Holliday-Segar formula.
- **Option B:** Similarly, without a specific value, we assume it's incorrect if it doesn't align with the calculated 42 mL/hour for a child weighing 10 kg.
- **Option D:** This option is incorrect if its value does not match 42 mL/hour, as calculated.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **Holliday-Segar formula** provides a straightforward method to estimate maintenance fluid needs in children: 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg, and 20 mL/kg for each kg thereafter. This is crucial for pediatric patients who require careful fluid management.

**Correct Answer:** B. 40-50 mL/hr (approximation might slightly vary, but closest to 42 mL/hour).