NBME Surgery Shelf Practice Test

Postoperative ileus happens mainly because the bowel’s propulsion is slowed by the combination of opioid use for pain control and the body’s response to surgical stress. Opioids bind to μ receptors in the enteric nervous system, which decreases acetylcholine release and dampens coordinated peristaltic contractions. This directly slows intestinal transit. At the same time, surgical manipulation of the gut triggers inflammation and increases sympathetic activity, further suppressing smooth muscle contractility and delaying movement of contents through the intestines. Together, these factors are the most common cause of a temporary ileus after surgery. Dehydration or electrolyte disturbances can worsen the situation, but they are not the primary driver in most patients. A mechanical obstruction presents with different signs (such as persistent severe pain, continuous vomiting, and no passage of gas or stool with abdominal x-ray findings suggesting blockage), rather than a generalized slowdown of motility.