**Core Concept**
Meningococci, specifically Neisseria meningitidis, possess various virulence factors that enable them to cause invasive meningococcal disease. These factors include outer membrane proteins, capsule, and other surface structures that facilitate adhesion, invasion, and evasion of the host immune response.

**Why the Correct Answer is Right**
The correct answer is the capsule, composed of polysaccharide, which is the primary virulence factor for meningococci. The capsule prevents phagocytosis by neutrophils and macrophages, allowing the bacteria to establish a foothold in the host. The capsule is also responsible for the organism's ability to cause meningitis and septicemia. The capsule is essential for the pathogenesis of meningococcal disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while outer membrane proteins like PorA and PorB are important for the adherence and invasion of meningococci, they are not the primary virulence factor.

**Option B:** This option is incorrect because while the lipopolysaccharide (LPS) of meningococci does contribute to the pathogenesis of the disease, it is not the primary virulence factor.

**Option C:** This option is incorrect because while the presence of certain genes can contribute to the virulence of meningococci, they are not the primary virulence factor.

**Clinical Pearl / High-Yield Fact**
The capsule of Neisseria meningitidis is the primary virulence factor and is the target for meningococcal vaccines. The capsule is composed of polysaccharide and is responsible for the organism's ability to cause meningitis and septicemia.

**Correct Answer:** D. The capsule is the primary virulence factor for meningococci.