She posed as a doctor, but was actually just a student.

posed 是动词 pose 的过去式。pose 在这里的意思是"假装、冒充"，指故意表现出某种身份或状态。pose 原意是"摆姿势"，引申为"假装、冒充"时通常与 as 连用，pose as...表示"假装成..."。