what is dermoid sinus?

this is a common problem in certain types of dogs...what exactlly is it?

It appears to be a tube in the neck of the dog that occasionally becomes clogged with hair (it has hairs growing inside it that shed just as hairs on the surface do).

http://www.upei.ca/cidd/Diseases/dermatology/dermoid%20sinus.htmThis will explain it. Looks like it is a defect inherited from the parents. It is there from birth, but usually not noticed until later.Good luck.