Sforno on Exodus 4:8/ספורנו על שמות ד׳:ח׳

1והאמינו לקול האות האחרון, from a medical science point of view the ability to heal tzoraat was a far greater miracle as it meant bringing back to life a limb which had already died ולקחת ממימי היאור, seeing that water is a basic liquid, one of the four basic raw materials earth is constructed of, converting this into blood without adding any additional ingredient to it was a most remarkable miracle.