Unable to Progress - A Rotting Subject

I am currently stuck on this quest. I found and cleared the corrupted ruin holding the corrupted mushroom/corrupted roots prior to speaking to Desma. When I returned with the purity potion, it still says, "You can't fathom a way to dispel it."

I didn't have this problem with the Carnute/was able to clear that part of the quest. I tried rebooting the game, which did not resolve.