**Core Concept**
Splenectomy is a surgical procedure that removes the spleen, an organ involved in filtering the blood, storing red blood cells, and producing antibodies. After splenectomy, patients are more susceptible to infections, particularly those caused by encapsulated bacteria, due to the loss of splenic function in filtering these pathogens.

**Why the Correct Answer is Right**
The most common infection after splenectomy is due to the increased risk of infections caused by encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitidis. This is because the spleen plays a crucial role in filtering these bacteria from the bloodstream. Without a functioning spleen, patients are more likely to develop infections from these pathogens, which can be severe and potentially life-threatening.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while infections from encapsulated bacteria are a concern after splenectomy, they are not the most common type of infection. Other types of infections, such as those caused by Staphylococcus aureus or Escherichia coli, are more common but not as directly related to the loss of splenic function.
* **Option B:** This option is incorrect because while urinary tract infections (UTIs) can occur after splenectomy, they are not the most common type of infection in this population. UTIs are more common in women and are related to urinary tract anatomy rather than splenic function.
* **Option C:** This option is incorrect because while pneumonia can occur after splenectomy, it is not the most common type of infection. Pneumonia is a possible complication of splenectomy, but it is not as directly related to the loss of splenic function as infections from encapsulated bacteria.

**Clinical Pearl / High-Yield Fact**
Patients who have undergone splenectomy should receive vaccination against encapsulated bacteria, including pneumococcus, Hib, and meningococcus, to reduce the risk of infection. This is a critical aspect of post-splenectomy care and should be emphasized to patients and their families.

**Correct Answer: D. Streptococcus pneumoniae**