Which of the following is a connective tissue defect in American Saddlebreds and Belgians?

Epitheliogenesis imperfecta is a congenital defect that affects the formation of epithelial tissue, leading to the absence or malformation of skin and mucous membranes. While this condition does have implications for the skin and outer layers of the body, it is not classified as a connective tissue defect, which primarily involves issues with the body's structural components such as tendons, ligaments, cartilage, and bone.

On the other hand, tendonitis, navicular disease, and osteochondritis dissecans are all conditions that result from issues related to connective tissue. Tendonitis refers to inflammation of a tendon, which is a type of connective tissue. Navicular disease impacts the navicular bone and surrounding tissues, while osteochondritis dissecans involves aberrant development of cartilage in the joint, affecting the underlying bone and joint health. These conditions are directly related to connective tissue abnormalities and are more commonly observed in certain breeds, including American Saddlebreds and Belgians.

Thus, when discussing connective tissue defects specific to these breeds, tendonitis, navicular disease, and osteochondritis dissecans would be more relevant than epitheliogenesis imperfecta.