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Serum sickness is a type of immune-mediated hypersensitivity reaction that occurs typically after exposure to certain medications, such as antitoxins, vaccines, or monoclonal antibodies. The onset of symptoms is generally associated with the time it takes for the immune system to recognize the foreign protein and mount an immune response, which, in the case of serum sickness, involves the production of antibodies that form immune complexes.

The average timeframe for the development of symptoms after exposure to the offending medication is commonly between 4 to 10 days. This period allows adequate time for the immune system to react. Patients may present with a variety of symptoms, including fever, rash, arthralgia, and lymphadenopathy, during this timeframe as immune complexes circulate and trigger inflammation.

Other options extend beyond this average period either too early or too late, which does not align with the typical clinical understanding of serum sickness timing. Therefore, the choice indicating 4 to 10 days is considered the most accurate in the context of serum sickness symptom onset.