**Core Concept**
Chloroquine is an antimalarial medication used for the treatment and prevention of malaria. The recommended dose of chloroquine varies based on the age and weight of the patient, as well as the severity of the disease.

**Why the Correct Answer is Right**
For children aged 4-8 years, the recommended dose of chloroquine phosphate is 5 mg/kg of body weight, not to exceed 300 mg. This dose is typically given once a week for one to two weeks. The correct dose is essential to prevent toxicity and ensure effective treatment of malaria. Chloroquine works by inhibiting the parasite's ability to digest hemoglobin, thereby preventing the parasite's growth and replication.

**Why Each Wrong Option is Incorrect**
**Option A:** 150 mg is too low for children aged 4-8 years, as it does not meet the recommended dose of 5 mg/kg.
**Option C:** 450 mg and **Option D:** 600 mg are excessive and may lead to toxicity, including nausea, vomiting, and in severe cases, cardiotoxicity and neurological symptoms.

**Clinical Pearl / High-Yield Fact**
When prescribing chloroquine, it is essential to consider the child's weight and adjust the dose accordingly to prevent toxicity. A commonly used formula for calculating the dose is 5 mg/kg/day, not to exceed 300 mg, for children aged 4-8 years.

**✓ Correct Answer: B. 300 mg**