House has realized that Rebecca may have neurocysticercosis (a tapeworm) from eating pork, something that would never have occurred to him if he still believed the patient was Jewish. That would explain why she reacted well to the steroids initially, but then got worse: tapeworms usually stay in the digestive system, but the eggs can pass into the bloodstream and then flourish anywhere, including in the brain.
This was literally the pilot episode of House MD.
https://en.wikipedia.org/wiki/Pilot_(House)
https://house.fandom.com/wiki/Pilot