Q. A 29-year-old man comes to the emergency department due to severe right knee pain and swelling, which gradually developed during the past 48 hours. He has no recent injuries or significant medical conditions. He is sexually active with multiple partners. He drinks 4-5 cans of beer daily and smokes one pack daily but does not use recreational drugs. He has a history of allergy to penicillin, which was treated as an outpatient. He also complains of pain during urination and urethral discharge from one week ago. Temperature is 37.8 degrees Celsius (100.0 degrees Fahrenheit), blood pressure is 121/70 mmHg, and pulse is 84/min. The left knee is swollen, erythematous, warm, and tender on examination. Both the active and passive range of motion is limited and painful. The remainder of the examination is unremarkable. The laboratory exam shows: ## Blood WBC: 13,250/mm3 * Neutrophils 68% * Lymphocytes 26% * Monocytes 4% ESR: 27mm/h CRP: 1.1 mg/dL ## Synovial fluid Color: Yellow Clarity: Opaque Viscosity: Low WBC 22,000/mm3 with 80% neutrophils Crystal: negative Gram stain: Positive for Gram-negative diplococci What is the best initial empiric antibiotic for immediate treatment?