**Core Concept**
The question is testing the understanding of potential complications related to subtotal gastrectomy, a surgical procedure for treating gastric cancer. The underlying concept is the anatomy and physiology of the gastrointestinal tract, particularly the relationship between the stomach and the duodenum.

**Why the Correct Answer is Right**
Bilious vomiting is a significant complication of subtotal gastrectomy due to the disruption of the normal anatomy of the upper gastrointestinal tract. During subtotal gastrectomy, the surgeon removes a portion of the stomach, which can lead to a disruption in the normal flow of bile from the liver into the small intestine. When the bile enters the stomach, it can cause vomiting, which may be bilious in nature. This complication is related to the altered anatomy and the potential for bile reflux into the stomach.

**Why Each Wrong Option is Incorrect**
**Option A:** Pneumothorax is a possible complication of thoracic surgery, not typically associated with subtotal gastrectomy. While a pneumothorax can occur during any surgical procedure, it is not a direct consequence of the surgery in question.

**Option B:** Constipation is not a direct complication of subtotal gastrectomy. In fact, patients who undergo subtotal gastrectomy may experience changes in bowel habits, including diarrhea or dumping syndrome, rather than constipation.

**Option C:** Hemolytic anemia is not directly related to subtotal gastrectomy. While patients undergoing surgery may experience changes in blood cell counts, hemolytic anemia is not a typical complication of this specific procedure.

**Clinical Pearl / High-Yield Fact**
It's essential for surgeons and anesthesiologists to be aware of the potential for bile reflux and vomiting after subtotal gastrectomy, as this can lead to dehydration and electrolyte imbalances. Prompt recognition and management of these complications are critical to prevent long-term consequences for the patient.

**✓ Correct Answer: D. Bilious vomiting**