**Core Concept**
The cardiothoracic ratio (CTR) is a radiological measurement used to assess the heart's size in relation to the chest cavity. In children, a normal CTR is essential for distinguishing between physiological and pathological cardiac enlargement.

**Why the Correct Answer is Right**
The cardiothoracic ratio in children is typically between 50-55%. This is because the heart size is proportionate to the chest cavity in pediatric patients. A CTR within this range indicates that the heart is not significantly enlarged. The American Heart Association recommends using the CTR to evaluate cardiac size in children, and a value of 50-55% is considered normal for this age group. The chest X-ray is a commonly used tool for assessing the CTR, and a normal value helps to rule out cardiac conditions such as hypertrophic cardiomyopathy.

**Why Each Wrong Option is Incorrect**
**Option A:** A CTR of 30-35% would be considered hypotrophic, indicating a small heart size, which is not a normal finding in children.

**Option B:** A CTR of 40-45% is also not within the normal range for children, as it suggests a slightly enlarged heart.

**Option D:** A CTR of 60-65% would indicate a significantly enlarged heart, which is not a normal finding in pediatric patients.

**Clinical Pearl / High-Yield Fact**
When interpreting the cardiothoracic ratio in children, it's essential to consider the patient's age, sex, and body size, as these factors can affect the normal range. A normal CTR in children can help to rule out cardiac conditions, but an abnormal value may require further evaluation with echocardiography or other imaging modalities.

**✓ Correct Answer: C. 50-55%. The cardiothoracic ratio in children is typically between 50-55%.**