Introduction
Blau Syndrome is an autoinflammatory disorder considered a human inborn error of immunity. It results from a heterozygous mutation in the NOD2/CARD15 gene on chromosome 16q12, inherited in an autosomal dominant pattern. There have been less than 100 reported mutations. The typical age of onset is under 5 years, and the condition is characterized by a triad of granulomatous arthritis, uveitis, and dermatitis.

Presentation of the case
The case involves a 5-year-old boy, conceived through in vitro fertilization with a 44-year-old mother. The pregnancy had complications like threatened abortion at 12 weeks and gestational diabetes in the fourth month. The boy had a history of treated developmental dysplasia of the hip and patent ductus arteriosus. In September 2020, he was hospitalized with symptoms of fever and erythematous maculopapular dermatosis on the limbs and chest. Six days earlier, he had experienced pharyngitis, but two COVID-19 tests (confirmed with PCR) came back negative. He was initially diagnosed with Incomplete Kawasaki disease versus Multisystem inflammatory syndrome in children and received extensive treatment with steroids, acetylsalicylic acid, intravenous immunoglobulin (three doses), cyclosporine, and tocilizumab (one dose) for 6 months. Following this treatment, the boy started experiencing episodic symptoms lasting one day every fifteen days, including generalized arthralgia, cheilitis, plantar erythema, and crampy abdominal pain. A genetic study confirmed the presence of a mutation in the NOD2 gene, which was consistent with Blau Syndrome.

Discussion
Blau Syndrome is a rare disease with varying clinical presentations, and the atypical features observed in this patient might have been influenced by the immunosuppressive treatment. The disease development is associated with the activation of the NF-κB pathway, which was likely inhibited at multiple levels due to the treatment administered to the patient.