how many days have elapsed since the last nitroprusside sodium prescription patient 10855 received in their current hospital visit?
select 1 * ( strftime('%j',current_time) - strftime('%j',prescriptions.startdate) ) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 10855 and admissions.dischtime is null ) and prescriptions.drug = 'nitroprusside sodium' order by prescriptions.startdate desc limit 1