The child who is at highest risk of contracting pertussis is the:

Rationale: A. A partially vaccinated 4-year-old is at highest risk for pertussis because incomplete vaccination provides suboptimal immunity, leaving the child vulnerable to infection. B. An infant born at term may have some passive maternal antibodies if the mother was vaccinated, offering partial protection. C. A child who came in contact with an infected person is at risk, but the degree of susceptibility depends on immunity status. A fully vaccinated child has significantly lower risk. D. A newborn whose mother received DTaP in the third trimester typically has transplacental antibodies, providing temporary protection during the first few months of life.