You are answering a USMLE-style medical question. Question: A 36-year-old man undergoes open reduction and internal fixation of a left femur fracture sustained after a motor vehicle collision. Three days after the surgery, he develops fever and redness around the surgical site. His temperature is 39.5°C (103.1°F). Physical examination shows purulent discharge from the wound with erythema of the surrounding skin. Wound culture of the purulent discharge shows gram-positive cocci in clusters. Treatment with oral dicloxacillin is initiated. Four days later, the patient continues to have high-grade fever, pain, and purulent discharge. Which of the following characteristics of the infecting organism best explains the failure to improve with antibiotic therapy? Options: A) Presence of an impenetrable outer membrane B) Ability to cleave β-lactam rings C) Development of efflux pumps D) Adaptation in binding proteins Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.