What structure in the horse's body is primarily affected by the condition caused by Cushing's Syndrome?

Cushing's Syndrome, also known as pituitary pars intermedia dysfunction (PPID) in horses, is primarily characterized by an overproduction of hormones resulting from a dysfunction of the pituitary gland. The pituitary gland regulates various hormonal functions in the body, including those that manage stress and metabolism. In horses with Cushing's, the pituitary gland produces excessive amounts of adrenocorticotropic hormone (ACTH), leading to increased production of cortisol from the adrenal glands. This hormonal imbalance is what defines the condition and its effects on the horse.

The other structures mentioned do not have a direct role in the pathology associated with Cushing's Syndrome. The heart may be indirectly affected due to changes in overall health and metabolic function, but it is not the primary site impacted by the condition. Similarly, while the kidneys and gastrointestinal tract can be involved in various health issues in horses, they are not primarily affected by or involved in the mechanism of Cushing's Syndrome. Thus, the focus on the pituitary gland highlights the root cause of the condition.