What Is The Cause Of Constant Anal Itching ?

I am a 28 yr old male and am in good physical health with one exception, my anus itches all the time and I don t know why. I noticed after I scratch I have a layer of dry, scaly skin under my finger nails and when the skin comes off, it gives off a very bad odor. Also, the itching can get so bad I draw blood, which makes it burn when I wash and is very painful when I wipe after using the restroom. What could cause all of this? It is also spreading. Whoom Ishould consult? Thanks in advance