In Situation 25.1, fecal samples were found to be negative when cultured. The next step in diagnosing the cause of illness would be

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Since the fecal samples were negative when cultured, the next step in diagnosing the cause of illness would be to perform microscopic examination of feces for oocysts, which are the reproductive forms of protozoan parasites like Cryptosporidium and Giardia. Oocysts are typically too small to be seen with the naked eye, so microscopic examination is necessary for diagnosis. Antibiotic therapy and blood cultures would not be appropriate for diagnosing protozoan infections. Muscle biopsy and microscopic examination of feces for viruses may also be useful in certain situations, but the presence of oocysts in the feces is the key factor in diagnosing protozoan infections.