Final answer:
Cyclosporine is the drug that causes pigmentation of the endothelial or Descemet's membrane in the eye.
Step-by-step explanation:
The drug that causes endothelial/Descemet's pigmentation is c) Cyclosporine. Cyclosporine is known to have various ocular side effects when administered systemically or topically as an ophthalmic agent. One of these side effects is pigmentation of the corneal endothelium or Descemet's membrane, which may be observed using a slit lamp examination by an ophthalmologist.
Endothelial cells are crucial in maintaining corneal transparency, and while Cyclosporine-induced pigmentation might be benign, it's essential for clinicians to monitor any changes in the cornea's health. The pigmentations are likely due to the drug's interactions with the corneal endothelium, although the exact mechanism is not fully understood.