"I would prepare to get a definitive diagnosis via a colonoscopy, because after failure of powerful antibiotics, I would suspect necrotic bowel. However, in my experience, I know that any colon ravaged like this for three days presents a high risk of perforation, so I would make sure I emphasized that in the informed consent. I would also involve an anesthesiologist for both complete patient immobility to reduce the likelihood of perforation and to maintain a surgery readiness should the need for emergency surgery prove necessary, such as colon resection of necrotic bowel."

Like another question, the one about a difficult intubation, this is a question that serves as a barometer for clinical common sense and basic medical judgment. Therefore, show them that you have that judgment. It is conventional medical wisdom that when a patient fails to respond to treatment based on a certain diagnosis, that you should doubt that diagnosis. Don't be afraid to be bold here with a drastic mid-course correction, because this is going to be a dead patient very soon!

"I would consider the therapy with the antibiotics a failure, so I would prepare to do what I needed to establish a correct diagnosis, which means a colonoscopy."