Database: scholar
Question: what keyphrase does Brian DeRenzi write about that gets most citations ?
SQL: select distinct t1.keyphrasename  ,  sum ( t3.numcitedby ) from paperkeyphrase as t2 join keyphrase as t1 on t2.keyphraseid  =  t1.keyphraseid join paper as t3 on t3.paperid  =  t2.paperid join writes as t4 on t4.paperid  =  t3.paperid join author as t5 on t4.authorid  =  t5.authorid where t5.authorname  =  "brian derenzi" group by t1.keyphrasename order by sum ( t3.numcitedby ) desc;