hi,
if you don't have an MR for a file then you should tick NO ,which  will mean you could only sell the file as Licensed (editorial) not as RF. yes in some way this limits chance of sale but editorial images has a different target buyers and as far as I know 70% of the files sold on Alamy are editorial images.
edited to add:
even if you believe the people in the picture are not recognizable you still need to tick no  but may be in this case you don't have to  add restriction to the file(editorial only restriction) but you need to bear in mind you 'd be doing it at your own risk. (my comment is obviously not a legal advice just my point of view)

good luck!

Thanks, but I don't understand why I need a model release for a hand injecting in an arm. Is this really neccessary at Alamy?

I see now,yes you are right but in this case instead of ticking YES for the question on MR I'd  tick 0 people in the photo photo thus you wouldn't be asked for an MR and offer your work as RM(if you prefer that)
these are just my thoughts may be you should contact Alamy for a better response.