Knowing you're adopted doesn't make you automatically know who your biological parents are. Or where they are. Or how to contact them.

Is really not that big a deal and many people don't accurately know their parents medical history or able to ask them. Generally the only reason is if you say yes, they might monitor some things closer. Or check for something earlier. But they're not gonna say screw you that condition is impossible because it doesn't run in your family that you know of. They still follow the symptoms.

False information regarding history will never harm you. Answering no won't have them automatically rule it out. Especially because they're aware bad communication, adoption etc is a thing.

And if you're really worried? Just tell them you have a history of everything. Yeah mum had this. Dad had that. Grandma said something about that once. Etc. It really doesn't matter outside of more attention given to it. You might slightly waste a little more time and resources. And maybe grandpa really did have that thing. My dads dad passed when he was a child. I don't know jack all about his history or what could have developed later in life.