**Core Concept**
A truncal vagotomy involves cutting the vagus nerve, which regulates various bodily functions including digestion, heart rate, and respiration. In the context of a duodenal ulcer, the procedure aims to reduce gastric acid secretion by interrupting the vagus nerve's stimulatory effect on the parietal cells in the stomach.

**Why the Correct Answer is Right**
The correct procedure to be performed alongside a truncal vagotomy is pyloroplasty or gastrojejunostomy. Pyloroplasty involves creating a new opening in the pylorus, the region of the stomach that connects to the small intestine, allowing food to pass more easily. This is necessary because a truncal vagotomy can cause the stomach to become a "bag" that fills with food, leading to stasis and potential complications. Gastrojejunostomy, on the other hand, involves creating a direct connection between the stomach and the small intestine, bypassing the pylorus altogether.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrectomy is incorrect because it involves removing a portion of the stomach, which is not a necessary step in a truncal vagotomy.
**Option B:** Cholecystectomy is incorrect because it involves removing the gallbladder, which is not related to the treatment of duodenal ulcers or the procedure of truncal vagotomy.
**Option C:** Gastroplasty is incorrect because it involves creating a new stomach pouch, which is not a necessary step in a truncal vagotomy.

**Clinical Pearl / High-Yield Fact**
A key consideration in performing a truncal vagotomy is the potential for post-operative dumping syndrome, which occurs when food passes too quickly from the stomach to the small intestine. This can be mitigated by performing a pyloroplasty or gastrojejunostomy to prevent food stasis.

**Correct Answer:** C. Gastrojejunostomy.