Which antibiotic was discovered by Alexander Fleming?

Penicillin is the antibiotic discovered by Alexander Fleming in 1928. Fleming observed that the mold Penicillium notatum produced a substance that inhibited the growth of bacteria, particularly Staphylococcus species. This marked a significant breakthrough in the field of microbiology and medicine, leading to the development of penicillin as the first true antibiotic and paving the way for the antibiotic era.

The discovery of penicillin was monumental because it provided a powerful tool for combatting bacterial infections, which had previously claimed countless lives. Penicillin's effectiveness against a wide range of bacterial infections dramatically changed the landscape of medical treatment and has saved millions of lives since its introduction.

Other antibiotics and treatments listed in the options, like sulfanilamide and prontosil, are important in their own right but have different origins and mechanisms of action. Sulfanilamide is a sulfa drug that was synthesized and used before penicillin but does not originate from a natural source like penicillin. Salvarsan was an early treatment for syphilis developed by Paul Ehrlich. While all these drugs played crucial roles in the history of medicine, Fleming's discovery of penicillin is particularly noteworthy for its profound impact on bacteriology and infectious disease treatment.