Which condition is a common allergic reaction seen in dogs?

Flea allergy dermatitis is indeed a common allergic reaction seen in dogs. This condition occurs when a dog is hypersensitive to flea saliva, which can cause intense itching, inflammation, and skin lesions. When a flea bites a dog, it injects saliva into the skin, leading to an allergic reaction in sensitive individuals. Symptoms typically include excessive scratching, biting at the skin, and hair loss, particularly around the tail and lower back.

In contrast, heartworm disease is a serious parasitic infection transmitted by mosquitoes, hip dysplasia is a genetic bone and joint disorder, and cataracts are an eye condition that leads to cloudiness in the lens. These conditions may affect dogs but do not stem from an allergic reaction, making flea allergy dermatitis the only option that fits the characteristics of an allergic reaction in this context.