Multiple myeloma initially presenting with oral mucosal neoplasm and concurrent cast nephropathy: one case report with a literature view oleh Zi-qing Gao, Jie Xiao, Li-juan Bian, Zhen-jian Xu, Zhen-lin Li, Jia-nan Su, Peng-wei Chen, Xiu-ju Wang, Qiong-qiong Yang, Min Feng