**Core Concept**
OPSI is a life-threatening infection that occurs in individuals who have undergone splenectomy, particularly those with asplenia or hyposplenism. It is characterized by an overwhelming response to encapsulated bacteria, leading to severe sepsis and high mortality rates. OPSI is a clinical entity that highlights the importance of the spleen in filtering pathogens from the bloodstream.

**Why the Correct Answer is Right**
The spleen plays a crucial role in filtering encapsulated bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis, from the bloodstream. Individuals with asplenia or hyposplenism are at increased risk of OPSI due to the lack of splenic function. The spleen's filtering mechanism involves the recognition of bacterial capsular polysaccharides by splenic macrophages, which then activate a robust immune response to eliminate the pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** OPSI is indeed characterized by an overwhelming response to encapsulated bacteria.
**Option B:** OPSI is a life-threatening infection that occurs in individuals with asplenia or hyposplenism.
**Option C:** OPSI is a clinical entity that highlights the importance of the spleen in filtering pathogens from the bloodstream.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to be aware of the increased risk of OPSI in patients with asplenia or hyposplenism, particularly those with a history of splenectomy. Vaccination against encapsulated bacteria, such as pneumococcal conjugate vaccine, is recommended for these patients to reduce the risk of OPSI.

**Correct Answer: D.**