Surgical Site Infection (SSI): This is the most common cause of redness, swelling, pain, and itching at a surgical site post-appendectomy. The presence of these symptoms, especially if they occur together, strongly suggests an infection.

Allergic Reaction to Sutures or Dressings: An allergic reaction could cause itching, redness, and swelling at the surgical site. This is less common than SSI but should be considered, especially if the patient has a history of allergies.
Hematoma or Seroma: These can cause swelling and pain at the surgical site. While they might not typically cause itching, they are common enough post-surgically to be considered in the differential diagnosis.
Wound Dehiscence: Although more severe, wound dehiscence (partial or complete separation of previously closed surgical incision) can present with pain, swelling, and possibly itching due to the exposure of underlying tissues and potential infection.

Necrotizing Fasciitis: A rare but life-threatening infection that requires immediate attention. It can present with severe pain, swelling, and redness, and the skin may feel warm or cool to the touch. The presence of itching is less typical but cannot rule out this diagnosis.
Abscess: A collection of pus that can develop in the wound, requiring drainage. An abscess can cause localized redness, swelling, pain, and sometimes itching due to the body's reaction to the infection.

Erythema Marginatum: A rare condition that could cause a rash around the wound site, potentially causing itching and redness. However, this is less directly related to the surgical site itself and more to a systemic reaction.
Contact Dermatitis: While not rare in general, in the context of post-appendectomy wound care, contact dermatitis from topical agents used in wound care could be considered a rare cause of itching and redness at the surgical site.