A 2 year old child is brought to the emergency department with a history of fever and vomiting. On examination he has neck rigidity. CSF examination shows polymorphs more than 2000/microL, protein 100 mg/dL, and glucose 10 mg/dL. Gram stain shows Gram negative coccobacilli. Culture shows growth of bacteria only on chocolate agar and not on blood agar. The causative agent is

Question

Image below highlights jugular foramen. Which of the following structures do not pass through the marked structure “A”?

Answer 1

The CSF findings, high polymorphs, high protein, and low glucose, tell us this is bacterial meningitis. The Gram stain and the culture behaviour narrow down the exact bug.

Neisseria meningitidis: this organism is a diplococcus, paired like coffee beans, not a coccobacillus, and it grows fine on plain blood agar. It does not need a special factor rich medium, so it does not match the clue.
Haemophilus influenzae: this is a small Gram negative coccobacillus that must have both factor X (hemin) and factor V (NAD) to grow. Chocolate agar is made by heating blood agar, which breaks open red cells and releases both factors. Plain blood agar keeps most of the V factor locked inside intact red cells, so this organism grows on chocolate agar but not on blood agar, exactly as described. It is also a well known cause of meningitis in toddlers who have not had the Hib vaccine.
Branhamella catarrhalis: also a diplococcus, it grows on ordinary blood agar without any special factor requirement, and it rarely if ever causes meningitis.
Legionella pneumophila: this bug needs buffered charcoal yeast extract agar with iron and cysteine to grow, not chocolate agar, and it causes a lung infection, not this meningitis picture.

Only Haemophilus influenzae fits both the coccobacillary shape on Gram stain and the growth only on chocolate agar.

Let's summarize:

Chocolate agar supplies factor X and factor V by lysing red cells during heating.
Haemophilus influenzae needs both factors, so it fails to grow on plain blood agar.
The Gram negative coccobacillary shape also points away from the diplococci Neisseria and Moraxella.

So the causative agent is Haemophilus influenzae, option 2.

Answer 2

The CSF findings, high polymorphs, high protein, and low glucose, tell us this is bacterial meningitis. The Gram stain and the culture behaviour narrow down the exact bug.

Neisseria meningitidis: this organism is a diplococcus, paired like coffee beans, not a coccobacillus, and it grows fine on plain blood agar. It does not need a special factor rich medium, so it does not match the clue.
Haemophilus influenzae: this is a small Gram negative coccobacillus that must have both factor X (hemin) and factor V (NAD) to grow. Chocolate agar is made by heating blood agar, which breaks open red cells and releases both factors. Plain blood agar keeps most of the V factor locked inside intact red cells, so this organism grows on chocolate agar but not on blood agar, exactly as described. It is also a well known cause of meningitis in toddlers who have not had the Hib vaccine.
Branhamella catarrhalis: also a diplococcus, it grows on ordinary blood agar without any special factor requirement, and it rarely if ever causes meningitis.
Legionella pneumophila: this bug needs buffered charcoal yeast extract agar with iron and cysteine to grow, not chocolate agar, and it causes a lung infection, not this meningitis picture.

Only Haemophilus influenzae fits both the coccobacillary shape on Gram stain and the growth only on chocolate agar.

Let's summarize:

Chocolate agar supplies factor X and factor V by lysing red cells during heating.
Haemophilus influenzae needs both factors, so it fails to grow on plain blood agar.
The Gram negative coccobacillary shape also points away from the diplococci Neisseria and Moraxella.

So the causative agent is Haemophilus influenzae, option 2.

Show Hint

The Correct Option is B

Solution and Explanation

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