In the absence of steroid use, when an ACTH stimulation test is performed and the serum cortisol level remains constant or is only mildly elevated (5%-10%), the finding is indicative of:

A. primary adrenal insufficiency
B. diabetes insipidus (DI)
C. syndrome of inappropriate antidiuretic hormone (SIADH)
D. hyperosmolar hyperglycemic nonketotic syndrome (HHNS)