Forget the noun, listen to the article.
Un is easily distinguished from une. That tells you what will follow in terms of number and gender.
Everybody is working so hard to hear some subtle difference in the noun when you already know what you need to know from the article by the time you get to the noun. .
By the way, please tell how widely used is "bitch" for the animal in English. Dictionaries put this meaning as first in the list. As well as in French.
I saw a same discussion (in Spanish).at http://duolingo.com/#/comment/77751
As told in the latter address, the more common slang translation in Spanish would be "zorra" for "chienne", since one generally does not have such an animal as pet, then one wouldn't use "zorra" in daily talks. However, it's needed a context, I think.
Bitch is routinely used to refer to female animals of select species by those professionally involved with animals as well as those who are serious hobbyists.
Outside of those circles it is seldom used that way.
Every native English speaker understands that meaning of the term, but avoid it because it has acquired such a pejorative tone from being misapplied to female humans.
Casually referring to your pet female dog with the term bitch would cause listeners to wonder about your feelings toward your pet. It would be seen as technically correct but rather cold (to say the least).
Du is a contraction of de le . Thus du lait is of the milk.
In English we have no comparable usage so we translate it as some.
Using le lait would mean not some milk in general but some particular milk because you are referring to the milk.
However in this instance Duo has chosen to translate du as the. I have no idea why. They certainly don't in any of the other instances of du that they have used so far. It has always been employed as some and students were encouraged to discard it at will when translating to English.