Which arsenic compound is administered via deep IM injection in the lumbar region?

Melsomine dihydrochloride is the correct answer because it is specifically used in veterinary medicine as a treatment for certain types of parasitic infections, particularly heartworm disease in dogs. Administered via deep intramuscular injection, particularly in the lumbar region, this compound effectively targets the heartworms. The deep IM injection helps ensure proper absorption and minimizes the risk of irritation at the injection site.

In contrast, the other compounds listed have different uses or routes of administration. Thiacetarsamide, for example, is also related to treatment of parasitic infections but does not typically involve deep IM injections. Arsenic trioxide, while used in some therapeutic contexts, is not used in such a manner for the same applications as Melsomine. Dimercaprol is primarily an antidote for heavy metal poisoning and is administered differently, also not aligning with the specific use of Melsomine dihydrochloride.