單項(xiàng)選擇題5歲男孩,面色蒼白半年入院,間有臍周痛,家住農(nóng)村,貧血貌。心率108次/分,律齊,心尖部聞及Ⅱ級(jí)收縮期雜音,腹軟,肝右肋下0.5cm,RBC4.1×10/L,Hb85g/L,WBC8.5×10/L,N0.5,L0.41,E0.09,RC0.02,MCV65fl,大便潛血(+)。引起本例貧血的原因是()

A.葉酸缺乏
B.鐵缺乏
C.維生素B缺乏
D.慢性炎癥
E.再生障礙性貧血