when was the last time that patient 73155 was prescribed with insulin during their last hospital encounter?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 73155 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'insulin' order by prescriptions.startdate desc limit 1