**Core Concept**
Intestinal transplantation is a complex surgical procedure that involves replacing a diseased or nonfunctioning small intestine with a healthy one. The post-transplant period is critical, and patients are at risk for various complications. The most common complications can be broadly categorized into surgical, immunological, and metabolic.

**Why the Correct Answer is Right**
The most common complication after intestinal transplantation is rejection of the transplanted intestine. This occurs when the immune system of the recipient identifies the transplanted tissue as foreign and mounts an immune response against it. Rejection can be acute or chronic and can lead to graft failure if not promptly treated. The risk of rejection is higher in the first few months after transplantation and can be managed with immunosuppressive medications.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection is a common complication after intestinal transplantation but is not the most common complication. Infections can occur due to the immunosuppressed state of the patient and can be caused by bacteria, viruses, or fungi.
**Option B:** Bile duct complications are a well-known issue after intestinal transplantation, but they are not the most common complication. Bile duct complications can lead to bile leakage, bile duct stenosis, or bile duct obstruction.
**Option C:** Anastomotic complications are a risk after intestinal transplantation, but they are not the most common complication. Anastomotic complications can include leakage, stricture, or obstruction at the site of intestinal anastomosis.

**Clinical Pearl / High-Yield Fact**
To reduce the risk of rejection after intestinal transplantation, patients are typically placed on a regimen of immunosuppressive medications that include calcineurin inhibitors, corticosteroids, and antiproliferative agents. Monitoring of serum levels of these medications and regular follow-up with the transplant team are essential to prevent rejection and minimize the risk of complications.

**Correct Answer:** D. Rejection of the transplanted intestine.