Donovan's solution has been used in veterinary medicine to treat chronic diseases of the skin. It is mercuric iodide mixed with:

• A. Arsenic triiodide
• B. Potassium iodide
• C. Sodium iodide
• D. Silver iodide

Answer: (A)

Donovan's solution is a historic topical mixture used in veterinary dermatology that combines mercuric iodide with an arsenic-containing iodide. The arsenic component is arsenic triiodide, which together with mercuric iodide gives the strong antiseptic/keratolytic effect sought for chronic skin conditions. The other iodide salts listed (potassium iodide, sodium iodide, silver iodide) are not part of this preparation, so arsenic triiodide is the correct match.