|Mrs. Hufnagel (Florence Halop) lays down the law.|
She had been taking four times the prescribed dose, but nonetheless demands a new physician who isn't trying to kill her. The duty falls on Dr. Jack Morrison (David Morse), whom Hufnagel recognizes as the one with the red knapsack and the kid who never says much on rounds. She questions Morrison about his medical education (which he obtained in Mexico), and then is horrified to learn that the doctor has ordered a barium enema for her: "N-O. Nobody touches the booty."
According to Wikipedia, phenylbutazone is no longer approved for human use in the United States, but is commonly prescribed to horses. Hufnagel was admitted with gastrointestinal bleeding, which can be a side effect...in horses.
It will turn out that Hufnagel is right to question Morrison's medical education; in the next episode ("Whistle, Wyler Works"), it will be revealed that Jack fast-tracked his medical degree, and is not actually qualified to practice medicine. He will have to redo medical school while doing his residency and raising his infant son by himself; this lasts into season four. Fortunately, his diagnostic skills improve considerably once he actually learns some medical knowledge.