GASTROENTEROLOGY

Understand the mechanisms of malabsorption in a patient with extensive ileal resection.

Explanation:

This is an excellent question because its answer stems from the basics of physiology.

3 main points to remember are

1. Surface area for absorption is reduced = short bowel syndrome
2. Bile salts undergo reabsorption in the ileum and their pool drops if reabsorption does not take place. These are necessary for micelle formation of fat and thus absorption of fat and fat soluble vitamins.
3. Vitamin B12 - which is is  normally absorbed in the distal ileum now cannot be absorbed because that part of the intestine has been removed.

This leads to diarrhea because of "1" above and fat malabsorption due to "2". We can also understand that vitamins A,D,E & K would not be absorbed properly. And megaloblastic anemia occurs due to "3".