**Core Concept**
Cellulitis is a type of bacterial infection of the skin and subcutaneous tissues, characterized by inflammation and tissue damage. It is typically caused by gram-positive cocci, such as Staphylococcus aureus and Streptococcus pyogenes, although gram-negative bacilli can also be involved.

**Why the Correct Answer is Right**
Cellulitis results from the spread of bacteria from a contiguous site of infection or through the bloodstream. The infection leads to the activation of the immune system, causing increased blood flow and permeability to the affected area, which results in the characteristic signs of erythema, warmth, swelling, and tenderness. The bacteria produce enzymes that break down tissue, leading to the release of histamine and other mediators that further exacerbate inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Cellulitis is typically not caused by fungal infections, which are more commonly associated with conditions like tinea pedis or onychomycosis.
**Option B:** Impetigo is a different type of skin infection that is characterized by the formation of golden crusted lesions, rather than the diffuse inflammation seen in cellulitis.
**Option C:** Erysipelas is a superficial form of cellulitis that typically involves the upper dermis and is more commonly associated with Streptococcus pyogenes.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate cellulitis from erysipelas, as the latter may require specific treatment with antibiotics that cover Streptococcus pyogenes.

**Correct Answer: A. Cellulitis is a bacterial infection of the skin and subcutaneous tissues.**