A 35-year-old Indian man with a history of recurrent Neisseria meningitidis and Neisseria gonorrhoeae infections presents to the immunology clinic. Serum complement levels are measured: C3 95 mg/dL (normal 80–160), C4 28 mg/dL (normal 15–45), C5 <1% (normal 5–10%), C6 <1%, C7 <1%, C8 <1%, C9 <1%. CH50 (total hemolytic complement) is undetectable. Which complement component deficiency best explains this clinical presentation, and what is the mechanism of increased susceptibility to Neisseria species?