A 28-year-old man from Delhi presents with recurrent pyogenic infections (Staphylococcus aureus, Streptococcus pneumoniae) and a history of two episodes of meningitis caused by Neisseria meningitidis. Physical examination reveals hepatosplenomegaly. Serum complement levels are normal, but serum bactericidal activity is absent. Which complement component deficiency is most likely responsible for his clinical presentation?