【名词&注释】
风湿性心脏病(rheumatic heart disease)、甲状旁腺功能减退症(hypoparathyroidism)、肺动脉瓣(pulmonary valve)、弱酸性(weak acidic)
[单选题]某弱酸性(weak acidic)药在pH为5时约90%解离,其pKa为( )
A. 6 B. 5 C. 4 D. 3 E. 2