**Core Concept**
OPSI (Overwhelming Post-Splenectomy Infection) is a life-threatening infection that occurs in individuals who have undergone splenectomy, particularly those with asplenic or hyposplenic conditions. The risk of OPSI is significantly increased in patients with certain underlying conditions, such as sickle cell disease or hereditary spherocytosis.

**Why the Correct Answer is Right**
The most common organism responsible for OPSI is **Streptococcus pneumoniae**. This bacterium is a encapsulated organism that requires the spleen to filter it out from the bloodstream. In the absence of a functioning spleen, S. pneumoniae can cause severe and rapidly progressing infections, including bacteremia, meningitis, and sepsis. The polysaccharide capsule of S. pneumoniae is a key virulence factor that allows it to evade the host's immune system.

**Why Each Wrong Option is Incorrect**
* **Option A:** Staphylococcus aureus is a common cause of skin and soft tissue infections, but it is not the most common cause of OPSI.
* **Option B:** Haemophilus influenzae type b (Hib) is a significant pathogen in children, but it is not the most common cause of OPSI in adults.
* **Option C:** Neisseria meningitidis is a leading cause of meningitis and sepsis, but it is not the most common cause of OPSI in patients with asplenic conditions.

**Clinical Pearl / High-Yield Fact**
Patients with asplenic or hyposplenic conditions should receive pneumococcal conjugate vaccine (PCV) and pneumococcal polysaccharide vaccine (PPSV) to reduce the risk of OPSI.

**Correct Answer: D. Streptococcus pneumoniae**