MCQ of the Day: 7 November 2026

The clinical picture (toddler meningitis with Gram-negative coccobacilli) plus classic satellitism on blood agar points to Haemophilus influenzae, which requires both hemin (X factor) and NAD (V factor). Sheep blood agar supplies inaccessible intracellular hemin and little free NAD; Staphylococcus aureus synthesises NAD and, by haemolysing red cells, liberates hemin, so H. influenzae colonies appear only in the immediate vicinity of the staphylococcus. Chocolate agar, in which red cells are already lysed and NAD is available, supports confluent growth without a feeder streak.

A. Catalase from S. aureus can protect some organisms from peroxide, but that is not the mechanism of Haemophilus satellitism and does not meet the dual X+V growth-factor requirement.

C. Beta-lactamase shielding (the “helper” phenomenon around resistant neighbours) is unrelated to this organism’s absolute nutritional dependence on X and V factors.

D. H. influenzae is a facultative organism, not an obligate anaerobe; oxygen tension is not why it fails as a lawn on blood agar.