## **Core Concept**
Acute sinusitis is an inflammation of the paranasal sinuses that lasts less than 4 weeks. The condition is often caused by infections, with the most common pathogens being viral, followed by bacterial. The microbiology of acute sinusitis helps guide antibiotic therapy.

## **Why the Correct Answer is Right**
The correct answer, **Streptococcus pneumoniae**, is one of the most common bacterial causes of acute sinusitis. This organism is a gram-positive coccus that frequently colonizes the nasopharynx and can cause invasive disease under certain conditions. Other common bacterial causes include *Haemophilus influenzae* and *Moraxella catarrhalis*.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Staphylococcus aureus* can cause sinusitis, but it is less commonly associated with acute sinusitis compared to *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Moraxella catarrhalis*. It is more commonly associated with skin and soft tissue infections.
- **Option B:** *Klebsiella pneumoniae* is more commonly associated with urinary tract infections and pneumonia, particularly in hospitalized patients or those with specific risk factors.
- **Option D:** *Pseudomonas aeruginosa* is more commonly associated with infections in immunocompromised individuals, cystic fibrosis, or hospital-acquired infections.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the most common causes of acute bacterial sinusitis are *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Moraxella catarrhalis*. Viral infections are also a common cause, but bacterial causes are often targeted with antibiotics.

## **Correct Answer:** .