A 28-year-old man from Delhi presents with purulent urethral discharge and dysuria for 3 days. He reports unprotected sexual contact with a new partner 5 days ago. On examination, the urethral meatus is inflamed with copious yellowish discharge. Gram stain of the urethral smear shows gram-negative diplococci within polymorphonuclear leukocytes. What is the most appropriate first-line treatment for this patient?