The time the syndrome takes to appear following the initial viral infection is debated, though it may develop between the first and second week. Epidemiological data suggest that recognition of the disease may typically be delayed by 2–6 weeks, and usually by 3–4 weeks. By the time of presention, children have often developed antibodies to SARS-CoV-2, but test negative for the virus at RT-PCR.The condition may match some or all of the diagnostic criteria for Kawasaki disease (i.e. the complete or incomplete/atypical subtypes), or for Kawasaki disease shock syndrome. It tends to affect all paediatric age groups, ranging from infancy to adolescence. It can also share clinical features with other paediatric inflammatory conditions, including toxic shock syndrome, and secondary haemophagocytic lymphohistiocytosis or macrophage activation syndrome. Coinfections with other pathogens have been recorded.Affected children always present with persistent fever. Other clinical features at presentation vary. In contrast to acute COVID-19, most children have gastrointestinal symptoms, such as diarrhoea, vomiting, and intense abdominal pain (sometimes severe enough to suggest appendicitis). Muscle pain and feelings of tiredness and general physical weakness are also very common. Some Kawasaki-like symptoms that may be present (especially in children under the age of 5) include mucosal changes around the mouth ("strawberry tongue", cracked lips, etc. ), red eyes (conjunctivitis without pus), widespread rash (consistent with leukocytoclastic vasculitis), red or swollen hands and feet, and enlarged lymph nodes. Chest or neck pain may also be present. Severe headache and altered mental state have been reported, along with various neurological disturbances.
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    {\displaystyle {\ce {3Fe + 4H2O->Fe3O4 + 4H2}}}
  
Under anaerobic conditions, ferrous hydroxide (Fe(OH)2) can be oxidized by water to form magnetite and molecular hydrogen.