instruction: when did patient 9241 have a microbiology test for the last time since 04/2105?
response: SELECT microbiologyevents.charttime FROM microbiologyevents WHERE microbiologyevents.hadm_id IN (SELECT admissions.hadm_id FROM admissions WHERE admissions.subject_id = 9241) AND STRFTIME('%y-%m', microbiologyevents.charttime) >= '2105-04' ORDER BY microbiologyevents.charttime DESC LIMIT 1