*begin to bit her mechanical pencil*
um...i thing who can be A....but the idea to directs the cells for herself sound more like a virus (changing-destroying the chromosomes and put her own with her decisions about the black and white things of all in the red blood cells) and a Shikieiki is more like a living thing, like a bacteria or a kind of parasite...(no,parasite no,because she is beneficious, she no is a lazy XD) no, A is not. nomnomnom.
B...no,we are in biology class no philosophy class...um,can be,but i will read the other ones.um...nom,nom...
C...interesting theory, but in the esqueme look another thing...i can`t see the Medicine toxines (the poison of the bacteria XD)...and...In PoFV Shikieiki fight against all important great "body defenses" of Gensokyo too (Reimu,Marisa,Sakuya,Youmu...and even Cirno XD) that sound like a Lupus...and you know,never is Lupus.XD...nomnom.
D is the answer,because the Yamatrocite look identical like her origin, intication of her ADN...and, like the heart cells (who can beat like a micro heart too), she judges and diriges the other cells correctly.XD
*marks D option with a completly destroyed mechanical pencil*