Henoch-Schönlein Purpura (HSP): This condition is a form of vasculitis that affects small blood vessels and is common in children. It often presents with a tetrad of symptoms: palpable purpura, abdominal pain, arthritis, and renal involvement. The perirectal rash could be part of the purpuric rash seen in HSP, and myalgia can occur due to the inflammatory nature of the disease.

Infectious Mononucleosis: Caused by Epstein-Barr virus (EBV), it can present with myalgia and rash, although the rash is not typically perirectal. However, some cases might have atypical presentations.
Kawasaki Disease: An acute febrile illness of childhood that includes symptoms like rash, fever, and lymphadenopathy. Myalgia can be present, and while the rash is not specifically perirectal, the disease's variability could lead to consideration in a differential diagnosis.
Erythema Multiforme: A skin condition characterized by target lesions, often triggered by infections or medications. It can present with a rash and systemic symptoms like myalgia, although the rash is typically more widespread than perirectal.

Meningococcemia: A severe infection caused by Neisseria meningitidis, which can present with a rash (often petechial or purpuric) and systemic symptoms like myalgia. Early recognition is crucial due to its high mortality rate if not promptly treated.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Severe skin and mucous membrane disorders, usually a reaction to medications or infections. While the rash is typically widespread, any child with a severe rash and systemic symptoms like myalgia should be evaluated for these conditions due to their potential for high morbidity and mortality.
Child Abuse: In cases of unexplained injuries or rashes, especially in sensitive areas like the perirectal region, child abuse must be considered and investigated appropriately.

Juvenile Dermatomyositis: An autoimmune disease characterized by muscle inflammation and skin rash. While it's rare, it could present with myalgia and a rash, although the rash is typically more characteristic (e.g., Gottron's papules).
Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by fever, neutrophilia, and tender erythematous skin lesions. It's rare in children and might not typically present with a perirectal rash, but it's a consideration in atypical cases with significant systemic symptoms.