The Quran and Exodus describe the same outward sign differently, but this does not by itself show that the Quran misunderstood the Bible. The key issue is whether Exodus requires the word "leprous" to mean an ordinary disease, and whether the Quran's phrase "without disease" is a contradiction or a theological clarification.
What the objection gets right
Exodus 4:6-7 plainly says that Moses' hand became affected with tsara'at, commonly translated "leprous" or "diseased," and that it was restored. A critic can therefore argue that the biblical sign is specifically disease followed by healing, whereas the Quran presents whiteness without harm:
"And draw your hand close to your side; it will come out white without disease - another sign."
Quran 20:22
On a straightforward reading, the two accounts do differ in emphasis. The Quran does not describe Moses as becoming leprous, nor does it narrate a healing from that condition. If one assumes that the English word "leprous" in Exodus must carry the full modern meaning of a disease, this appears to be a contradiction.
What the Quran actually says
The Hebrew term tsara'at is not identical to modern medical "leprosy." In the Hebrew Bible it can refer to various visible skin conditions and is also used in contexts involving clothing and buildings. The phrase "white as snow" describes appearance, but does not by itself settle the medical nature of the sign.
The Quran likewise does not portray the hand as merely producing a harmless glow. It says that the hand comes out visibly white, and presents this as a divine sign:
"Put your hand into your garment; it will come out white without disease. And draw your hand close to your side to be free from fear. These are two proofs from your Lord to Pharaoh and his chiefs."
Quran 28:32
Classical Muslim commentators understood the whiteness as a real, extraordinary transformation, while taking "without disease" to distinguish it from a harmful illness. The point is not that nothing happened to the hand, but that its whiteness was a miraculous sign rather than a pathological affliction. The Quran's wording also stresses the sign's purpose: confirming Moses' mission and removing his fear.
What this does and does not establish
It does establish that the Quran gives a different description from the conventional English reading of Exodus. Muslims should not conceal that difference. But it does not establish that the Quran copied a corrupted story or made an obvious factual mistake. The biblical term is broader than modern "leprosy," and Exodus itself describes an instantaneous miraculous change and restoration, not an ordinary medical case.
Thus, the objection succeeds only if its translation and interpretation of tsara'at are treated as unquestionable. The Quran's account is a deliberate clarification: Moses' hand became extraordinarily white, but not through a harmful disease. Whether one accepts that clarification ultimately depends on whether one accepts the Quran as a later divine revelation, not on a decisive contradiction in the wording alone.