What is the primary characteristic of paronychia?

The primary characteristic of paronychia is a purulent infection of the proximal and lateral nail folds. This condition typically occurs when bacteria or fungi infect the skin surrounding the nail, leading to symptoms such as redness, swelling, and pus formation. The infection predominantly affects the tissue around the nail and is commonly associated with trauma or irritation to the nail area, which allows pathogens to enter and proliferate.

The other options describe different medical conditions that do not pertain to paronychia. For instance, a superficial fungal infection within the epidermis describes a different type of infection that affects the skin more broadly, such as tinea. The mention of a bacterial infection resembling tinea cruris refers to a specific type of fungal infection that affects the groin area, which is also not related to the nails. Lastly, an acute infection of the central nervous system caused by tetanus bacillus is entirely outside the scope of nail or skin infections, as it deals with a serious systemic condition. Thus, the defining symptom of paronychia is accurately captured by the description of a purulent infection surrounding the nail folds.