cla·rith·ro·my·cin 
(klə-rĭth′rə-mīsĭn)
(klə-rĭth′rə-mīsĭn)n.
A macrolide antibiotic, C38H69NO13, used to treat a variety of bacterial infections, especially of the respiratory tract.
[Probably C(H)LA(MYDIA) (the causative agent of a type of pneumonia targeted by the drug) + alteration of (E)RYTHROMYCIN.]