when patient 56502 received a catheter tip-iv microbiology test for the last time?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 56502 ) and microbiologyevents.spec_type_desc = 'catheter tip-iv' order by microbiologyevents.charttime desc limit 1