A 31-year-old man has gonorrhea. He has no drug allergies, but he recalls that a few years ago while in Africa he had acute hemolysis following use of an antimalarial drug. The physician is concerned that the patient has an accompanying urethritis due to C trachomatis, though no cultures or enzyme tests have been performed. Which of the following drugs is most likely to be effective against gonococci and to eradicate C trachomatis in this patient?