**Core Concept**
Meningococci (Neisseria meningitidis) and gonococci (Neisseria gonorrhoeae) are two species of Gram-negative diplococci. The key difference between them lies in their pathogenicity, virulence factors, and the diseases they cause.

**Why the Correct Answer is Right**
Meningococci are the causative agents of meningitis, a severe infection of the meninges, the protective membranes surrounding the brain and spinal cord. This is due to the presence of surface proteins like pili, which facilitate adhesion to and invasion of host cells. In contrast, gonococci primarily cause sexually transmitted infections, such as gonorrhea. The main difference in their virulence factors is the presence of the capsule in meningococci, composed of polysaccharide, which is not found in gonococci.

**Why Each Wrong Option is Incorrect**
**Option A:** While both meningococci and gonococci are Gram-negative, this characteristic is not unique to meningococci and does not explain their primary difference.
**Option B:** Both species are capable of causing severe infections, but this does not address the key difference in their virulence factors and pathogenicity.
**Option C:** The presence of a capsule in meningococci is indeed a key virulence factor, but this option does not provide a complete explanation of the differences between the two species.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that meningococcal infections can rapidly progress to life-threatening sepsis and meningitis, and prompt diagnosis and treatment are crucial to prevent complications. The introduction of conjugate vaccines has significantly reduced the incidence of meningococcal disease.

**Correct Answer:** C.