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The choice that indicates which age group represents over 50% of childhood tuberculosis (TB) cases is the group of children under 5 years old. This age group is particularly vulnerable to TB, largely due to their developing immune systems, which makes them more susceptible to infections.

In children, especially those under the age of 5, the risk of severe illness and complications from TB is greater, and they are at a higher risk of exposure, as they often live in close quarters with family members or caregivers who may have active TB. The incidence of TB in pediatric populations tends to decrease as children grow older, as their immune systems mature and they typically have reduced environmental exposure to the bacterium responsible for TB.

Therefore, the understanding of TB epidemiology highlights that children under 5 account for a significant percentage of TB cases, reflecting the need for targeted screening and preventive measures in this particularly vulnerable demographic.