Rats have some anatomical peculiarities which may baffle a vet who is only used to dogs and cats.
They have a large, knobbly caecum (a cul-de-sac at the beginning of the large intestine, containing bacteria used to break down cellulose) which can be mistaken for an internal tumour.
They do not have a gall-bladder.
The right lung of a rat has four lobes, but the left lung has only one. If your vet is not expecting this, it can make chest X-rays hard to interpret.
Males do not have even rudimentary nipples (though they do have some mammary tissue).
Females have a sort of rudimentary penis called the genital papilla. They urinate through this, rather than through an opening at the entrance of the vagina as most mammals do - so a prolapsed uterus can be snipped off from the outside and stitched at the vagina, without blocking the urethra.