when did patient 1798 last had the maximum value of mch in their first hospital visit?
select labevents.charttime from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1798 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'mch' ) order by labevents.valuenum desc, labevents.charttime desc limit 1