Which of the following is a hereditary immunological disease in Arabians?

The correct answer is CID, which stands for Combined Immunodeficiency. This is a hereditary condition particularly found in Arabian horses and is caused by a genetic defect that affects the immune system. Horses affected by CID are unable to produce adequate amounts of immunoglobulins, which are crucial for fighting off infections. As a result, affected foals are more susceptible to infections and may not survive beyond a few months of age without intervention.

CID specifically represents a genetic disorder, distinguishing it from the other options listed. Equine Infectious Anemia, for example, is caused by a virus and is not inherited. Thoroughbred immuno-deficiency is not a recognized condition in the horse population, and while Hereditary Equine Regional Dermal Asthenia affects the skin and is inherited, it does not pertain to the immune system like CID does. Thus, CID is the only option that correctly identifies a hereditary immunological disease specific to Arabians.