Learn Italian: Hai le ginocchia sporche.

Yes, the singular is il ginocchio (masculine). However, several body parts in Italian have a completely irregular plural: they change to feminine and end in -a. So, the masculine il ginocchio becomes the feminine plural le ginocchia.

How would I say "You have a dirty knee" (singular)?

You would go back to the regular masculine singular forms: Hai un ginocchio sporco. Notice how the adjective sporco drops the "h" and changes to match the masculine singular noun!

Why does the adjective sporche have an "h" in it?

The base word is sporco (dirty), which has a hard "k" sound. Because ginocchia is feminine plural, the adjective must end in -e. To keep that hard "k" sound before an "e", Italian adds an "h". Without the "h", it would be spelled sporce and pronounced with a soft "ch" sound like in "cheese".

The English translation says "Your knees", but the Italian says Hai le ginocchia (You have the knees). Why?

In Italian, it is much more natural to use the verb avere (to have) and a definite article (le) when describing a temporary condition of someone's body parts. Instead of translating literally and saying "Your knees are dirty" (Le tue ginocchia sono sporche), Italians naturally phrase it as "You have the dirty knees".