**Core Concept**
Isoniazid is a first-line antitubercular agent that requires careful dosing in pediatric patients due to its narrow therapeutic index and risk of hepatotoxicity. The ideal dose is crucial to balance efficacy with safety, especially in children.

**Why the Correct Answer is Right**
The recommended dose of Isoniazid in children is 10-15 mg/kg body weight per day, administered once daily. This dose is based on the World Health Organization's guidelines for the treatment of tuberculosis in children. The dose should not exceed 300 mg per day to minimize the risk of hepatotoxicity. The pediatric dose is calculated based on the child's weight, and the medication should be given under the supervision of a healthcare provider to ensure accurate dosing.

**Why Each Wrong Option is Incorrect**
**Option A:** 5 mg/kg/day
This dose is too low and may not be effective in treating tuberculosis in children, increasing the risk of treatment failure.

**Option B:** 20 mg/kg/day
This dose is too high and may increase the risk of hepatotoxicity in children, which can be fatal.

**Option C:** 10 mg/kg/day, administered twice daily
While this dose is closer to the recommended dose, administering Isoniazid twice daily may increase the risk of hepatotoxicity due to the accumulation of the drug in the liver.

**Clinical Pearl / High-Yield Fact**
When prescribing Isoniazid to children, always calculate the dose based on the child's weight, and ensure that the dose does not exceed 300 mg per day to minimize the risk of hepatotoxicity.

**Correct Answer: A. 10-15 mg/kg body weight per day**