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pron·to·sil | \ ˈprän-tə-ˌsil 
\
\Medical Definition of prontosil
: any of three sulfonamide drugs:
a : a red azo dye C12H13N5O2S that was the first sulfa drug tested clinically
— called also prontosil rubrum
b : sulfanilamide
c : azosulfamide
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