What Causes Reddened, Itchy Patch-like Spot Behind Knee?

On the back of my knee, I have a reddened, itchy patch-like spot about the size of a silver dollar. It has been there around 6 months now. I went to a dermatoligest about 10 days ago, and he said it did not look like psoriasis. He did not do a scraping but instead gave me steriod and bacterial salve. The spot is still there but does not itch. Shouldn't the redness be gone by now? Does it sound like psoriasis if it is nowhere else on my body? Dennis.