**Core Concept**
The pathogenesis of *Haemophilus influenzae* (H. influenzae) involves various virulence factors, including the capsular polysaccharide, which plays a crucial role in its ability to cause invasive disease. The capsular polysaccharide is composed of a specific antigenic determinant that helps the bacterium evade the host immune system.

**Why the Correct Answer is Right**
Option C is incorrect because the capsular polysaccharide is not responsible for the virulence of H. influenzae. Instead, the bacterial capsule is composed of a polysaccharide made of a specific antigenic determinant, but it is not the primary virulence factor. The primary virulence factor of H. influenzae is the outer membrane protein P6, also known as the IgA protease, which helps the bacterium adhere to and invade host cells. The capsular polysaccharide, on the other hand, is essential for the bacterium's ability to evade the host immune system by inhibiting phagocytosis.

**Why Each Wrong Option is Incorrect**

**Option A:** H. influenzae requires factor X (hemin) and V (nicotinamide adenine dinucleotide, NAD) for growth, but this statement is true and not the correct answer.

**Option B:** H. influenzae can present as meningitis in children less than 2 months of age, and this statement is true. In fact, H. influenzae type b (Hib) is a leading cause of bacterial meningitis in this age group.

**Option D:** H. influenzae is a common cause of invasive disease, including meningitis, but this statement is not the correct answer because the question asks what is "all true except."

**Clinical Pearl / High-Yield Fact**
H. influenzae type b is a leading cause of bacterial meningitis in children under the age of 2, and the introduction of Hib conjugate vaccines has significantly reduced the incidence of this disease.

**✓ Correct Answer: C. Capsular polypeptide protein is responsible for virulence**