**Core Concept**
Splenectomy, or removal of the spleen, leads to an increased susceptibility to infections due to the spleen's role in filtering the blood and storing lymphocytes. The spleen is also responsible for removing encapsulated bacteria from the bloodstream, such as Streptococcus pneumoniae and Haemophilus influenzae. Without a spleen, patients are more prone to infections, especially those caused by these encapsulated organisms.

**Why the Correct Answer is Right**
The correct answer is related to the spleen's role in removing encapsulated bacteria. In a splenectomized patient, the lack of splenic function impairs the body's ability to clear these organisms from the bloodstream, leading to an increased risk of infection. The spleen's lymphoid tissue also plays a role in the immune response, and its removal can impair the body's ability to mount an effective response to infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the spleen is not primarily responsible for removing non-encapsulated bacteria from the bloodstream. While the spleen does have some role in filtering the blood, its primary function is to remove encapsulated organisms.

**Option B:** This option is incorrect because the spleen is not primarily responsible for removing viruses from the bloodstream. The spleen's role in removing viruses is not as well-defined as its role in removing encapsulated bacteria.

**Option C:** This option is incorrect because the spleen is not primarily responsible for removing fungi from the bloodstream. While the spleen does have some role in filtering the blood, its primary function is to remove encapsulated bacteria.

**Option D:** This option is incorrect because the spleen is not primarily responsible for removing protozoa from the bloodstream. The spleen's role in removing protozoa is not as well-defined as its role in removing encapsulated bacteria.

**Clinical Pearl / High-Yield Fact**
In patients who have undergone splenectomy, it is essential to administer vaccinations against encapsulated organisms, such as pneumococcal and meningococcal conjugate vaccines, to reduce the risk of infection.

**Correct Answer:** D. Protozoa