**Core Concept:** The concept being tested is the localization of specific gastrointestinal hormones and their actions within the gastrointestinal tract. The pancreas secretes two hormones, gastrin and secretin, which play a role in gastrointestinal motility and secretion. Gastrin stimulates gastric acid secretion, while secretin promotes bicarbonate secretion in the duodenum.

**Why the Correct Answer is Right:** Secretin is a hormone released by the G cells in the duodenum in response to the presence of acidic chyme (digested food) in the duodenum. It travels through the portal circulation to the anterior pituitary gland and acts on the S cell of the anterior pituitary gland, leading to the release of oxytocin and ACTH. Secretin plays a crucial role in maintaining pH of the duodenum to facilitate digestion and absorption of essential nutrients.

**Why Each Wrong Option is Incorrect:**

A. Gastrin is not mentioned in the question, but it is important to note that gastrin is released by chief cells in the stomach and acts on G cells in the duodenum to stimulate gastric acid secretion, which is the opposite of secretin's action.

B. This option is incorrect as this hormone is secreted by the G cells in the duodenum in response to acidic chyme, not present at the transpyloric level.

C. This answer is incorrect as it refers to G cells, which secrete gastrin, not secretin. Secretin is released by duodenal G cells in response to acidic chyme.

D. This answer is wrong as it describes somatostatin, a hormone secreted by D cells in the pancreas and acts on the duodenum to inhibit gastrin secretion, not secretin.

**Clinical Pearl:** Secretin and gastrin are two hormones that work together to maintain the acidic and alkaline environments within the stomach and duodenum, respectively, for optimal digestion and absorption of nutrients. Any imbalance in their secretion can lead to gastrointestinal disorders like peptic ulcer, which is a consequence of excessive gastric acid secretion.