**Core Concept:** Vaccines are biological preparations that stimulate the immune system to produce immunity to specific pathogens or conditions. The Indian Academy of Pediatrics (IAP) recommends certain vaccines for children up to two years of age.

**Why the Correct Answer is Right:** The correct answer, D, refers to the measles vaccine, which is typically administered at around 12 months of age as part of the Expanded Programme on Immunization (EPI) schedule. The other options, A, B, and C, are part of the standard EPI schedule and are essential for children up to two years of age, aiming to protect infants and young children from preventable diseases.

**Why Each Wrong Option is Incorrect:**

A. Polio vaccine: Polio is a viral infection caused by the poliovirus, and vaccination against it is crucial.

B. Hepatitis B vaccine: Hepatitis B is a viral infection that affects the liver, and vaccination helps prevent severe liver diseases.

C. Pneumococcal vaccine: Pneumococcal disease is caused by Streptococcus pneumoniae and vaccination helps in preventing severe infections like pneumonia and meningitis.

**Why Option D is Right:** D, the measles vaccine, is not typically given until around 12 months of age, after which it is included in the EPI schedule. Before that age, children are protected against measles through maternal antibodies, which are passed from the mother to the child during pregnancy and delivery. However, these antibodies usually wane by the age of six months, making measles vaccine necessary after that age.

**Correct Answer:** D. Measles vaccine