The accidental discovery of which substance by Alexander Fleming in 1928, after observing a mold inhibiting bacterial growth, led directly to the development of the first widely used antibiotics, revolutionizing medicine?

💡 Explanation:

Scottish biologist Alexander Fleming discovered penicillin in 1928 when he noticed that a mold (Penicillium notatum) had contaminated a petri dish and killed the surrounding bacteria. This discovery marked the beginning of the antibiotic age.