You are answering a USMLE-style medical question. Question: A 26-year-old male presents to his primary care physician with complaints of burning with urination, penile discharge, and intermittent fevers. A urethral smear shows gram negative diplococci within white blood cells. The organism grows well when cultured on Thayer-Martin agar. The patient is prescribed a course of ceftriaxone and the infection resolves without further complication. One year later, the patient returns with the same infection. Which of the following best explains this lack of lasting immunity? Options: A) Exotoxin release B) Antigenic variation C) Polysaccharide capsule D) Bruton's agammaglobulinemia Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.