**Core Concept**
Infectious disease control and quarantine measures are crucial to prevent the spread of tuberculosis (TB) and other respiratory diseases. Children below 6 years are particularly vulnerable to TB due to their developing immune system and higher susceptibility to severe forms of the disease.

**Why the Correct Answer is Right**
Children below 6 years are at higher risk of developing severe TB and are more likely to transmit the disease to others. In a sputum-positive patient, the risk of transmission is higher, and isolating young children is a critical measure to prevent the spread of disease. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) recommend isolating children under 6 years of age in close contact with a sputum-positive TB patient.

**Why Each Wrong Option is Incorrect**
**Option A:** Pregnant women are not necessarily at higher risk of contracting TB, and quarantine measures may not be as stringent for this group.
**Option B:** Old people may have developed immunity to TB through exposure or vaccination, and quarantine measures may not be as critical for this age group.
**Option C:** Children above 6 years are generally less susceptible to severe TB and may have developed some level of immunity through exposure or vaccination.

**Clinical Pearl / High-Yield Fact**
In TB control, children under 6 years of age are considered a high-risk group for disease transmission, and quarantine measures should be implemented to prevent the spread of disease.

**✓ Correct Answer: D. Children below 6 years**