what is the monthly maximum urea nitrogen value for patient 18726 in 2103?
select max(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18726 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'urea nitrogen' ) and strftime('%y',labevents.charttime) = '2103' group by strftime('%y-%m',labevents.charttime)