Final answer:
The worsening condition after penicillin administration is likely due to antibiotic resistance, such as MRSA, or the development of a superinfection after broad-spectrum antibiotic use.
Step-by-step explanation:
The most likely cause of the worsening condition after administration of penicillin is antibiotic resistance. Over time, bacteria like Staphylococcus aureus can develop resistance to antibiotics, including penicillins and their derivatives. This is evidenced by the case descriptions wherein patients did not respond to initial antibiotic treatments, such as oxacillin, and required alternative treatment courses with different, more potent antibiotics like vancomycin. Some bacteria evolve to be methicillin-resistant S. aureus (MRSA) or form biofilms on medical devices, making it much harder to treat the infections they cause. Furthermore, when potent broad-spectrum antibiotics are used, they can sometimes lead to a superinfection, which is a secondary infection that may occur due to the disruption of normal flora.