Thanks for all your help. Fortunately I do not have psoriasis. I guess this nail is brittle as looking closely it is very thin in the area where it cracked. I do recall when I trimmed the nail back, as I reached the area of the fungus, the underside of where it cracked was pretty much rotten (not sure if right word but best way to explain it) causing it to be so hollow. Also the crack did not go all the way back to the cuticle. It's only in the hollow area. I did file some more last night as oe side of the crack was sticking up and would get caught on my socks. Will see what happens in the next days.