Dropsy is caused from a bacterial infection of the kidneys, causing fluid accumulation or renal failure. The fluids in the body build up and cause the fish to bloat up and the scales to protrude.
Funfact: People mistake pearlscales for having droopsy based on way their scales look naturally.
This is a pearlscale. This what Pearlscale's scales look like. They kinda look like droopsy