Final answer:
For a severe duodenal ulcer leading to suspected perforation and peritonitis, the most immediate nursing intervention upon prescription is to insert a nasogastric (NG) tube to decompress the stomach and prevent further irritation of the abdominal cavity.
Step-by-step explanation:
When a patient with a severe duodenal ulcer complains of severe pain spreading over the entire abdomen, indicative of a potential perforation and resultant peritonitis, the most immediate nursing intervention, upon medical prescription, would be Insert a nasogastric (NG) tube into the stomach.
This is to decompress the stomach by removing stomach contents and gas, thus preventing further leakage into the abdominal cavity and minimizing the risk of increased peritonitis. This immediate intervention is critical while preparing for the next necessary steps, which often include emergency surgery to repair the perforation.