how much docusate sodium was first prescribed during their last hospital visit to patient 42396?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 42396 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'docusate sodium' order by prescriptions.startdate asc limit 1