A 6-year-old unimmunised child develops rapidly progressing meningitis. CSF Gram stain shows gram-negative coccobacilli. Blood and CSF cultures are inoculated on chocolate agar supplemented with X (hemin) and V (NAD) factors and satellite growth is observed near Staphylococcus streak. Which organism is responsible, and what is the capsular serotype associated with the highest virulence in children?

• A Neisseria meningitidis — gram-negative diplococcus requiring chocolate agar but not X and V factors
• B Haemophilus ducreyi — requires X factor only; gram-negative coccobacillus causing chancroid
• C Haemophilus parainfluenzae — requires V factor only; not associated with invasive meningitis
• D Haemophilus influenzae type b (Hib) — requires both X (hemin) and V (NAD) factors; satellite growth around S. aureus confirms X+V dependence; type b capsule (polyribosyl ribitol phosphate, PRP) is the most virulent serotype ✓

Explanation

Haemophilus influenzae requires both X factor (hemin, heat-stable) and V factor (NAD, heat-labile) for growth. Staphylococcus aureus secretes V factor (NAD), so H. influenzae satellites form around the staph streak on blood agar. Chocolate agar (heated blood releasing both X and V factors) supports primary growth. The type b capsule (polyribosyl ribitol phosphate, PRP) is the most antiphagocytic and virulent serotype, previously causing 95% of invasive Hib disease (meningitis, epiglottitis) in children. Hib conjugate vaccine has dramatically reduced invasive Hib disease. H. ducreyi requires only X factor; H. parainfluenzae requires only V factor.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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