Final answer:
The bacterial pathogen most likely causing a skin infection requiring drainage in an unvaccinated boy with a varicella infection is either Staphylococcus aureus or Streptococcus pyogenes.
Step-by-step explanation:
If an unvaccinated boy with varicella infection now has a skin lesion requiring drainage, the most likely bacterial pathogens would be Staphylococcus aureus or Streptococcus pyogenes. These bacteria commonly cause secondary infections in skin lesions due to the breach in skin integrity that occurs from the chickenpox pustules. For instance, in the case of impetigo, which is a bacterial skin infection that can co-occur in skin compromised by varicella, lesions can be caused by S. aureus, S. pyogenes, or both. These bacteria can invade open wounds and cause skin infections that might produce pus and require drainage.