What substance was discovered by Sir Alexander Fleming that significantly advanced modern medicine?

The correct answer is significant because Sir Alexander Fleming is credited with the discovery of penicillin in 1928, which was the world's first true antibiotic. This groundbreaking discovery had a profound impact on the field of medicine, as penicillin effectively kills or inhibits the growth of bacteria, leading to successful treatments of various bacterial infections. It marked the beginning of antibiotic therapy, transforming the way infections were treated and significantly reducing mortality rates from bacterial diseases.

While antibiotics refer to a broader class of medications used against bacteria, penicillin is specifically the first and one of the most important examples, establishing the prototype for many other antibiotics developed later. Cortisone and vaccines play crucial roles in medicine as well, but they serve different purposes; cortisone is primarily used as an anti-inflammatory and immunosuppressive medication, while vaccines are developed to prevent infectious diseases rather than treat them. Penicillin's discovery laid the foundation for the antibiotic revolution, making it essential in the context of medical advancement.