In the 1950s, many pneumonia patients were treated with the new antibiotic penicillin and showed initial improvement, but after a few weeks, they relapsed with infections that no longer responded to the drug. What is the most probable cause of this occurrence?

• A. The patients had been reinfected by different pneumococcal strains that were inherently penicillin-insensitive.
• B. The patients' immune systems had mounted a response that neutralized penicillin and made it ineffective.
• C. The infecting bacteria had acquired or selected for penicillin resistance during treatment.
• D. The patients' relapses were due to stopping supportive care measures like bed rest and nutrition rather than antibiotic failure.

Answer: (C)