My baby is taking penicillin for tonsillitis. after 48 hours he started showing a measel like rash should i stop giving him the penicillin?

Rash: It would be best. Also, consider looking for mononucleosis.

Depends: If the doc confirmed strep by testing there is a possibility a rash could be from the strep. Strep rashes are flat to the skin, red & have a sandpaper texture. A doc visit could separate a drug reaction rash from a strep rash. Penicillin can produce a drug rash but ~1/3 of kids with mono sore throat will get a purplish to red rash on amoxicillin.