**Core Concept**
The spleen plays a crucial role in immune function, particularly in filtering the blood and storing lymphocytes. After splenectomy, patients are at an increased risk of infections, especially those caused by encapsulated bacteria. The absence of the spleen impairs the body's ability to clear these pathogens from the bloodstream.

**Why the Correct Answer is Right**
Pneumococcus (Streptococcus pneumoniae) is an encapsulated bacterium that is commonly found in the nasopharynx. In the absence of the spleen, patients are more susceptible to pneumococcal infections, which can range from mild to life-threatening. This is because the spleen is responsible for filtering out encapsulated bacteria, such as pneumococcus, from the bloodstream. Without the spleen, these bacteria can evade the immune system and cause infections.

**Why Each Wrong Option is Incorrect**
**Option B:** Meningococcus (Neisseria meningitidis) is also an encapsulated bacterium, but it is not the most common cause of post-splenectomy infections. While meningococcal infections can be severe, they are relatively rare compared to pneumococcal infections.

**Option C:** Staphylococcus is a genus of bacteria that can cause a wide range of infections, but it is not typically associated with post-splenectomy infections. Staphylococcal infections are more commonly seen in patients with skin or soft tissue infections.

**Option D:** Corynebacterium is a genus of bacteria that can cause a variety of infections, but it is not commonly associated with post-splenectomy infections. Corynebacterial infections are more often seen in patients with respiratory or skin infections.

**Clinical Pearl / High-Yield Fact**
It's essential to note that patients with a history of splenectomy should receive pneumococcal vaccination to reduce their risk of pneumococcal infections. This is a critical aspect of preventive care in patients without a spleen.

**✓ Correct Answer: A. Pneumococcus**