**Question:** The commonest organism causing cellulitis is:

A. Staphylococcus aureus
B. Streptococcus pyogenes
C. Escherichia coli
D. Klebsiella pneumoniae

**Core Concept:** Cellulitis is a clinical entity characterized by inflammation of the skin and underlying subcutaneous tissues, typically caused by bacterial infections.

**Why the Correct Answer is Right:** Staphylococcus aureus is the most common cause of cellulitis, particularly in the superficial dermal and subcutaneous layers. It is a gram-positive, coagulase-positive cocci that colonizes the skin and mucous membranes, leading to skin infections.

**Why Each Wrong Option is Incorrect:**

B. Streptococcus pyogenes (Group A Streptococcus): While Streptococcus pyogenes can cause cellulitis, it is less common than Staphylococcus aureus.

C. Escherichia coli: E. coli primarily causes urinary tract infections, gastrointestinal infections, and rarely skin infections due to its anatomical location and lifestyle as a commensal gut bacterium.

D. Klebsiella pneumoniae: Similar to E. coli, K. pneumoniae is primarily involved in respiratory and urinary tract infections, not cellulitis.

**Why the Correct Answer is Right:** Staphylococcus aureus is the most common cause of cellulitis due to its ability to invade the skin and subcutaneous tissues, causing tissue destruction and inflammation. It is often present on the skin as a commensal bacterium and can cause local infections with subsequent spread to deeper tissues.

**Clinical Pearl:** In cases of cellulitis, it is essential to consider the causative organism and its usual habitat to guide appropriate antibiotic therapy and target treatment.

**Correct Answer:** A. Staphylococcus aureus