**Core Concept**
After splenectomy, patients are at increased risk of infection due to the absence of the spleen's immune function, particularly against encapsulated bacteria. This is because the spleen plays a crucial role in filtering the blood, storing red blood cells, and acting as a key component of the immune system, especially against encapsulated organisms.

**Why the Correct Answer is Right**
The correct answer is Streptococcus pneumoniae, an encapsulated bacterium that is highly prevalent in the environment. The spleen's function in filtering the blood and responding to encapsulated organisms is crucial for preventing pneumococcal infections. After splenectomy, the patient is more susceptible to invasive pneumococcal disease due to the lack of this immune function. The spleen's role in opsonizing and removing encapsulated bacteria, such as S. pneumoniae, is compromised, making patients more prone to severe infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while Haemophilus influenzae type b (Hib) is an encapsulated bacterium, it is not the primary cause of infection after splenectomy. Hib infections are more common in children.

**Option B:** This option is incorrect because Escherichia coli (E. coli) is a gram-negative bacterium that is not typically associated with increased risk of infection after splenectomy.

**Option C:** This option is incorrect because Staphylococcus aureus is a gram-positive bacterium that can cause a range of infections, but it is not specifically associated with an increased risk after splenectomy.

**Clinical Pearl / High-Yield Fact**
After splenectomy, patients should receive vaccinations against encapsulated bacteria, including pneumococcal conjugate vaccine (PCV) and meningococcal conjugate vaccine (MenACWY), to reduce their risk of infection.

**Correct Answer: D. Streptococcus pneumoniae**