**Question:** Commonest cause of cellulitis is

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

**Correct Answer:** B. Staphylococcus aureus

**Core Concept:** Cellulitis is a type of bacterial skin infection that affects the dermis and subcutaneous tissues. It presents with redness, warmth, swelling, and tenderness of the affected area. Cellulitis is typically classified as superficial or deep, depending on the extent of infection.

**Why the Correct Answer is Right:** Staphylococcus aureus, a common skin commensal, is the most common cause of cellulitis, particularly superficial cellulitis. This bacterium can cause cellulitis through two mechanisms:

1. Direct invasion: S. aureus can invade the skin and underlying tissues, leading to localized infection.
2. Indirect invasion: S. aureus can cause damage to the skin barrier, allowing other pathogens to invade and contribute to cellulitis.

**Why Each Wrong Option is Incorrect:**

A. Streptococcus pyogenes (Group A beta-hemolytic Streptococcus) is less commonly associated with cellulitis compared to S. aureus.

B. Staphylococcus epidermidis is a normal skin flora and is not typically responsible for cellulitis unless there is a compromised skin barrier or underlying pathology.

C. Escherichia coli (E. coli) is a common cause of gastrointestinal infections and urinary tract infections, not cellulitis.

D. Klebsiella pneumoniae is a common cause of pneumonia, urinary tract infections, and other respiratory infections, but is less commonly associated with cellulitis compared to S. aureus.

**Clinical Pearl:**

Understanding the common pathogens involved in cellulitis is crucial for accurate diagnosis and appropriate treatment. Staphylococcus aureus is the most common cause of cellulitis, while Streptococcus pyogenes is less frequently involved. The other options (S. epidermidis, E. coli, and K. pneumoniae) are typically associated with different clinical scenarios, such as gastrointestinal or urinary tract infections.

**Correct Answer Explanation:** Staphylococcus aureus is the most common cause of cellulitis, particularly in the superficial layers of the skin and underlying tissues. The bacterium can cause cellulitis through direct invasion or indirect invasion when the skin barrier is compromised or when an underlying pathology facilitates bacterial entry. This understanding is essential for effective management and prevention of cellulitis, as appropriate antibiotics can be prescribed based on the identified causative pathogen.