Which of the following is the most common organism isolated in acute bacterial rhinosinusitis in immunocompetent adults?

Explanation

## Microbiology of Acute Bacterial Rhinosinusitis ### Most Common Pathogens **Key Point:** Streptococcus pneumoniae is the single most common organism isolated in acute bacterial rhinosinusitis (ABRS) in immunocompetent adults, followed closely by non-typeable Haemophilus influenzae and Moraxella catarrhalis. ### Epidemiology of ABRS Pathogens | Organism | Frequency | Clinical Notes | |----------|-----------|----------------| | *Streptococcus pneumoniae* | 25–35% | Most common; often causes severe disease | | *Haemophilus influenzae* (non-typeable) | 20–30% | Second most common | | *Moraxella catarrhalis* | 10–15% | Less virulent; often in mixed infections | | *Pseudomonas aeruginosa* | 5–10% | Immunocompromised, cystic fibrosis, chronic sinusitis | | Anaerobes | 5–10% | Polymicrobial; associated with dental disease | ### High-Yield Facts **High-Yield:** The "Big Three" pathogens (S. pneumoniae, H. influenzae, M. catarrhalis) account for ~70% of culture-positive ABRS cases. S. pneumoniae is the single most frequent isolate and is associated with more severe presentations. **Clinical Pearl:** Pseudomonas aeruginosa becomes a significant pathogen in chronic sinusitis, immunocompromised states, and cystic fibrosis—not in uncomplicated acute disease in immunocompetent hosts. ### Mnemonic **SHMP** — *Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Pseudomonas* (in order of frequency in acute vs. chronic disease).