what is patient 18678's monthly maximum mchc level in 2104?
select max(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18678 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'mchc' ) and strftime('%y',labevents.charttime) = '2104' group by strftime('%y-%m',labevents.charttime)