A 25-year-old female presents with the acute onset of cessation of lactation. She delivered her first child several months ago & has been breast-feeding since then. She reports that she has not menstruated since the delivery. She also says that lately she has been tired & has been feeling cold all of the time. Laboratory workup reveals a deficiency of ACTH & other anterior pituitary hormones. What is the most likely cause of this patient's signs & symptoms?