Final answer:
Corneal edema observed under a prism ballast toric soft lens is likely due to the lens being too tight, restricting tears and oxygen flow.
Step-by-step explanation:
The probable cause of corneal edema being observed inferiorly under a prism ballast toric soft lens is that the lens is too tight. When a lens is too tight, it can restrict the flow of tears and hinder the exchange of oxygen between the contact lens and the cornea. This lack of oxygen supply can result in corneal edema.