**Core Concept**
Splenectomy, or the surgical removal of the spleen, can lead to various immediate complications due to the spleen's role in filtering the blood and its rich vascular supply. The most common immediate complication of splenectomy is related to the loss of this vital organ's function.

**Why the Correct Answer is Right**
The most common immediate complication of splenectomy is **infection**. This is because the spleen acts as a filter for the blood, removing pathogens and other foreign particles. Without the spleen, patients are more susceptible to infections, particularly those caused by encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. This increased risk of infection is due to the loss of splenic function in removing these pathogens from the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** Hemorrhage. While hemorrhage is a possible complication of splenectomy, it is not the most common immediate complication. Hemorrhage may occur due to the loss of vascular supply to the spleen, but it is not as prevalent as infection.

**Option B:** Thrombocytosis. Thrombocytosis, or an increased platelet count, can occur after splenectomy due to the loss of splenic sequestration of platelets. However, this is not an immediate complication and typically occurs several days to weeks after the surgery.

**Option C:** Hypersplenism. Hypersplenism, characterized by the removal of normal blood cells from the circulation, is a possible complication of splenectomy. However, this is not the most common immediate complication, and its onset is typically delayed.

**Clinical Pearl / High-Yield Fact**
Patients undergoing splenectomy should receive prophylactic antibiotics and vaccinations against encapsulated bacteria to reduce the risk of infection. This should include vaccination against Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis.

**Correct Answer: D. Infection**