###HUMAN: what is the number of patients whose drug code is eryt250 and lab test fluid is cerebrospinal fliuid (csf)?, ###ASSISTANT: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id INNER JOIN lab ON demographic.hadm_id = lab.hadm_id WHERE prescriptions.formulary_drug_cd = "ERYT250" AND lab.fluid = "Cerebrospinal Fluid (CSF)"