A pаtient presents with symptоms оf hypercоrtisolism. Hypercortisolism is confirmed through elevаted ACTH аnd elevated cortisol lab tests. The patient undergoes a pituitary MRI and no lesion is present. The patient is given an additional test where their CRH levels are measured via inferior petrosal sinus sampling (IPSS). CRH is shown to be highly elevated. What is the most likely pathology?