A 30-year-old man has developed fever, chills, and neck stiffness. Cerebrospinal fluid shows gram-negative diplococci. He has had a past episode of sepsis with meningococcemia. Select the most likely immunologic deficiency.

Patients who have a deficiency of one of the terminal components of complement have a remarkable susceptibility to disseminated Neisseria infection, particularly meningococcal disease. This association with meningococcal disease is related to the host inability to assemble the membrane attack complex-a single molecule of complement components that creates a discontinuity in the bacteria's membrane lipid bilayer. The complement deficiency results in inability to express complement-dependent bactericidal activity.