È così stupida da credere a una cosa simile.

[morbrorper]

English Original Sentence

She’s so stupid to believe such a thing.

Is da the correct preposition? I would naively have used a.

[mike-lima]

You are right. You can use “da credere”, but it means “She is so stupid she even believes such a thing”.
“È così stupida a credere una cosa simile” means “She is so stupid for believing such a thing”.