One by One - Season 1 Episode 2 : Nature's Great Masterpiece 1987

Donald and Maurice are called to the zoo when a female elephant is in extreme pain in her jaw, and is getting more and more debilitated and dangerous. Donald and Maurice discover that the elephant has severe arthritis and there just aren't the drugs to deal with it, and are forced to euthanase her. Donald is left in a black mood as he doesn't consider that he can help any of the animals at the zoo, with his limited knowledge.