**Core Concept**
Splenectomy, the surgical removal of the spleen, can lead to various complications due to the spleen's role in filtering the blood, storing red blood cells, and participating in the immune response. The most common complication of splenectomy is related to the body's inability to filter out old or damaged red blood cells, leading to an increased risk of certain infections.

**Why the Correct Answer is Right**
The correct answer is **B. Infections, particularly encapsulated bacteria**. This is because the spleen plays a crucial role in filtering out encapsulated bacteria, such as Streptococcus pneumoniae and Haemophilus influenzae, from the bloodstream. Without a spleen, patients are more susceptible to infections caused by these bacteria. This is due to the lack of splenic macrophages, which are responsible for removing these encapsulated bacteria from the blood.

**Why Each Wrong Option is Incorrect**
* **Option A:** While thrombocytosis (an increase in platelet count) can occur after splenectomy, it is not the most common complication.
* **Option C:** Portal vein thrombosis is a potential complication of splenectomy, particularly in patients with portal hypertension, but it is not the most common complication.
* **Option D:** Pancreatitis can occur after splenectomy, especially if the splenic vessels are ligated too close to the pancreas, but it is not the most common complication.

**Clinical Pearl / High-Yield Fact**
Patients who have undergone splenectomy should receive vaccinations against encapsulated bacteria, such as pneumococcal and meningococcal conjugate vaccines, to reduce their risk of infection.

**Correct Answer: B. Infections, particularly encapsulated bacteria**