**Cellulitis Pathophysiology**

**Core Concept**
Cellulitis is a bacterial infection of the skin and subcutaneous tissues, typically caused by Streptococcus and Staphylococcus species. It is characterized by erythema, edema, warmth, and tenderness of the affected area.

**Why the Correct Answer is Right**
The most common causative organisms of cellulitis are Streptococcus pyogenes (Group A beta-hemolytic streptococci) and Staphylococcus aureus. These bacteria invade the skin through breaks in the epithelial barrier, such as cuts or scrapes, and spread through the dermal lymphatics. The resultant inflammation and tissue damage lead to the clinical manifestations of cellulitis.

**Why Each Wrong Option is Incorrect**
* **Option A (Haemophilus influenzae):** While Haemophilus influenzae can cause cellulitis, particularly in children with underlying conditions such as sickle cell disease, it is not the most common causative organism.
* **Option B (Escherichia coli):** Escherichia coli is a gram-negative rod that typically causes urinary tract infections or gastrointestinal infections, rather than cellulitis.
* **Option C (Pseudomonas aeruginosa):** Pseudomonas aeruginosa is a gram-negative rod that can cause a variety of infections, including skin infections, but is not the most common cause of cellulitis.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with cellulitis, it is essential to consider the possibility of underlying conditions, such as diabetes or immunocompromised states, which may increase the risk of infection.

**Correct Answer: C. Staphylococcus aureus.**