**Question:** All of the following are true regarding Haemophilus influenzae except -

A. Haemophilus influenzae is a Gram-negative bacterium.
B. Haemophilus influenzae is a common cause of pneumonia and meningitis.
C. Haemophilus influenzae is susceptible to amoxicillin.
D. Haemophilus influenzae is a common cause of sinusitis.

**Correct Answer:**

**Core Concept:** Haemophilus influenzae (H. influenzae) is a common Gram-negative bacterium that can cause various infections in humans. It is known to be a causative agent of pneumonia, meningitis, and sinusitis. However, the bacterium has been largely eradicated due to widespread vaccination programs.

**Why the Correct Answer is Right:**

Amoxicillin is a penicillin-based antibiotic that targets Gram-positive bacteria, not Gram-negative bacteria like Haemophilus influenzae. Since H. influenzae is a Gram-negative bacterium, it is resistant to amoxicillin, making option C incorrect.

**Why Each Wrong Option is Incorrect:**

A. Haemophilus influenzae is indeed a Gram-negative bacterium, so this option is correct.
B. Although H. influenzae can cause pneumonia, it is not the commonest cause of pneumonia in children, as the commonest cause is Streptococcus pneumoniae. This option is incorrect.
D. Haemophilus influenzae is a common cause of sinusitis, but it is not the only bacterium causing sinusitis. Other organisms like Moraxella catarrhalis, Streptococcus pneumoniae, and Staphylococcus aureus can also cause sinusitis, making this option incorrect.

**Clinical Pearl:** Haemophilus influenzae is a noteworthy bacterium that causes a range of infections, including pneumonia and meningitis. However, the widespread use of vaccination has led to a decrease in its incidence. When treating infections caused by H. influenzae, it is essential to consider other pathogens that can cause similar symptoms and choose the appropriate antibiotic therapy accordingly.