A child presents with a maculopapular rash on the extremities and trunk. Petechiae is found on the trunk and mucous membranes. Lab tests show a Neisseria infection and the physician is concerned about meningococcemia. Which of the following is the most likely?

Correct Answer: B

Rationale: C5-C9 deficiency impairs the membrane attack complex, increasing susceptibility to Neisseria infections like meningococcemia.

Correct Answer: A

Rationale: Interferon gamma (IFN-γ) is the primary cytokine for activating macrophages, enhancing their phagocytic and microbicidal activity.

Correct Answer: A

Rationale: CD40L deficiency prevents class switching from IgM to other isotypes, resulting in hyper-IgM syndrome.

Correct Answer: C

Rationale: The MMR vaccine is live attenuated and contraindicated in severely immunocompromised patients due to the risk of vaccine-derived infection.

Correct Answer: A

Rationale: An epitope is the specific region on an antigen recognized and bound by an antibody.