Final answer:
The second dose of the MMR vaccine is given at 4 to 6 years of age to ensure lifelong protection and to boost the immune response.
Step-by-step explanation:
The second dose of the measles, mumps, and rubella (MMR) vaccine is recommended to be given at 4 to 6 years of age for several reasons. Firstly, the immune response to the first dose of the vaccine may not be sufficient to provide lifelong protection against these diseases. Secondly, the second dose helps to boost the immune response and increase the level of protective antibodies in the body. Lastly, giving the second dose at this age ensures that children are vaccinated before they enter school, where they may be exposed to these contagious diseases.