**Core Concept**
The spleen plays a crucial role in the immune system, particularly in filtering the blood and storing lymphocytes. Its removal, or splenectomy, can lead to an increased susceptibility to infections, especially those caused by encapsulated bacteria.

**Why the Correct Answer is Right**
The most common post-splenectomy infection is pneumococcal sepsis, primarily caused by *Streptococcus pneumoniae*. This is because the spleen is essential for filtering out encapsulated bacteria, including pneumococci, from the bloodstream. Without a spleen, patients are more prone to developing severe infections due to these bacteria. The increased risk is particularly evident in children and young adults who have undergone splenectomy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although *Haemophilus influenzae* type b (Hib) is an encapsulated bacterium, it is not the most common cause of post-splenectomy infections. This is because Hib vaccination has significantly reduced its incidence in many countries.
* **Option B:** *Staphylococcus aureus* is a common pathogen, but it is not typically associated with post-splenectomy infections. The increased risk of infections after splenectomy is more closely linked to encapsulated bacteria.
* **Option C:** *Escherichia coli* is a common cause of urinary tract infections and other types of infections, but it is not the most common post-splenectomy infection.

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

**Correct Answer:** C.