I don't think it's wrong, i just think that "lampa" should also be accepted... without egy
There are sentences where the Hungarian noun can stand without an article while the English translation requires one. But this is not a sentence, nor is there any context, so why would you omit it? A and egy are perfect equivalents here.
Why is the answer accepted without the indefinite article on other similar excercises such an "egy férfi" = "man" (without "a")? How is this one different?
Well, Hungarian keyboard layout has them.. :) You should also be able to download Hungarian input on your mobile.
A is used if the next word starts with something that is not a vowel, az is used when the next word starts with a vowel, like a,e,i,o,u you get it, egy can be used in both and also means 1