**Core Concept**
Post splenectomy infections are a significant concern in patients who have undergone splenectomy, a surgical procedure to remove the spleen. The spleen plays a crucial role in filtering the blood, storing red blood cells, and acting as a reservoir for lymphocytes, which are essential for the immune system. Without a functioning spleen, patients are more susceptible to infections, particularly those caused by encapsulated bacteria.

**Why the Correct Answer is Right**
The correct answer is related to the common causes of post splenectomy infections, which include encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitidis. These bacteria have a polysaccharide capsule that protects them from phagocytosis by macrophages and neutrophils, which are the primary immune cells that the spleen helps to defend against. The spleen's removal disrupts this defense mechanism, making patients more vulnerable to infections caused by these bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Streptococcus pneumoniae is a common cause of post splenectomy infections, particularly pneumonia and meningitis. The polysaccharide capsule of this bacterium is a key virulence factor that allows it to evade the immune system.

**Option B:** This option is incorrect because Haemophilus influenzae type b is another common cause of post splenectomy infections, particularly meningitis and septicemia. The polysaccharide capsule of this bacterium is a key virulence factor that allows it to evade the immune system.

**Option D:** This option is incorrect because Neisseria meningitidis is a common cause of post splenectomy infections, particularly meningitis and septicemia. The polysaccharide capsule of this bacterium is a key virulence factor that allows it to evade the immune system.

**Clinical Pearl / High-Yield Fact**
Patients who have undergone splenectomy are at increased risk of infections caused by encapsulated bacteria, and it is essential to administer prophylactic antibiotics, such as penicillin or amoxicillin, for a period of time after surgery to reduce this risk.

**Correct Answer:** D.