Question

A 6-week-old infant presents with bacterial meningitis. Which organism is the most likely cause?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Haemophilus influenzae
• D. Listeria monocytogenes

Hint:

Which organism makes us add ampicillin to empirical therapy in this age band?

Answer 1

The Correct Option is D

Step 1: Anchor on the age group. At 6 weeks the infant is at the tail end of the neonatal/early-infancy window, where the pathogen spectrum differs from older children. The classic neonatal-to-early-infancy triad is Group B Streptococcus, Escherichia coli, and Listeria monocytogenes (acquired largely from maternal genital/GI flora).

Step 2: Compare with the offered options. Group B Strep and E. coli are not listed, so among the four choices the organism that belongs to this early-infancy group is Listeria monocytogenes.

Step 3: Why the others are less likely at this age. • S. pneumoniae and H. influenzae become the dominant causes only after about 2-3 months as maternal antibody wanes; they are uncommon at 6 weeks. • S. aureus meningitis is usually post-neurosurgical, post-traumatic, or related to a focus/shunt, not spontaneous early-infancy meningitis.

Clinical pearl: Listeria is the reason empirical neonatal meningitis therapy adds ampicillin to a cephalosporin - cephalosporins do not cover Listeria.

Key fact: In a 6-week-old (early infancy), among the given options Listeria monocytogenes is the most likely meningitis pathogen.