Doctors refer difficult cases for specialist care; one such case was referred yesterday.

refer 是动词，意思是“转交、转诊”，指将病人或病例交给更专业的医生或机构处理。 referred 是 refer 的过去分词形式，在句中表示被动，意思是“被转诊的”，即该病例已经被提交给专科医生处理。