## **Core Concept**
Cellulitis is a bacterial infection of the skin and subcutaneous tissues, typically involving the dermis and subcutaneous fat. It is characterized by redness, swelling, warmth, and pain in the affected area. The most common organisms responsible for cellulitis are bacteria.

## **Why the Correct Answer is Right**
The correct answer, **Streptococcus pyogenes** and **Staphylococcus aureus**, are the most common bacteria implicated in cellulitis. These bacteria can enter the skin through cracks or breaks, leading to infection. Streptococcus pyogenes, also known as Group A beta-hemolytic streptococcus (GABHS), and Staphylococcus aureus, often methicillin-resistant (MRSA), are commonly isolated from cellulitis lesions.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain fungi can cause skin infections, they are less commonly associated with cellulitis compared to bacterial causes.
- **Option B:** This option might be incorrect based on the specifics of the organism listed, as the question seems to point towards common bacterial causes.
- **Option D:** This could be incorrect if it refers to a less common cause or an organism not typically associated with cellulitis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that cellulitis is often treated empirically with antibiotics that cover both streptococcal and staphylococcal species. In areas with high MRSA prevalence, antibiotics effective against MRSA, such as vancomycin or trimethoprim/sulfamethoxazole, may be chosen. A critical clinical correlation is that patients with recurrent cellulitis may benefit from measures to prevent future episodes, such as addressing chronic edema.

## **Correct Answer:** B. Streptococcus.