**Core Concept**
Diphtheria, Tetanus, and Pertussis (DTP) vaccination is a crucial component of childhood immunization, protecting against serious bacterial infections. The booster dose given to 5-6 years old children is aimed at maintaining immunity and preventing diseases caused by these pathogens.

**Why the Correct Answer is Right**
The booster dose of DTP vaccine is essential for maintaining long-term immunity against diphtheria, tetanus, and pertussis. This is because the initial doses given at 6-8 weeks and 10-12 weeks of age provide short-term protection, but wane over time. Booster doses help to refresh the immune system's response, ensuring continued protection against these infections. The diphtheria toxin is produced by Corynebacterium diphtheriae, while tetanus toxin is produced by Clostridium tetani. Pertussis, also known as whooping cough, is caused by Bordetella pertussis.

**Why Each Wrong Option is Incorrect**
**Option A:** Measles, Mumps, and Rubella (MMR) vaccine is given to children at 9-12 months and 16-18 months of age, not at 5-6 years.
**Option B:** Hepatitis B vaccine is given in a series of doses starting at birth and is not typically given as a booster dose at 5-6 years.
**Option C:** Influenza vaccine is seasonal and given annually to protect against the flu virus, not as a booster dose for DTP.

**Clinical Pearl / High-Yield Fact**
The DTP booster dose is crucial for maintaining immunity in children, as the initial doses provide short-term protection that wanes over time.

**Correct Answer: C. Pertussis.