Composition/perspective - your angle of approach to this shot is very good. Low, but not too low that the dog appears flat. Your subject is very centered, perhaps placing him off to one side or more towards a corner would make it a stronger image. The subject itself is strong enough that applying the rule of thirds would not have hurt the image. The negative space also would not have hurt if it was weighted to one side. The face appears a bit soft and the paws appear slighty fuzzy. Perhaps application of USM or a different focus would eliminate that.
Lighting - overall the lighting is well done. A few areas seem a bit blown out - from the top of the head to the middle of the back for example. You start to lose the definition of his back in the brightness and there azre areas where the subject is indistinguishable from the background. Control of the shadows is well done. Enough to keep the subject well defined in most areas, not too much to give dark spots.
Color - light on white obviously by definition does not contain a lot of color variation. The skin tone and spots showing on the subject really help to keep the white coat from getting totally lost in the background.
Challenge requirements - you obviously have met the challenge very well. You have a strong subject who seems to not mind posing for you.
Overall/my opinion - this is a very pleasing portrait. It could be a stronger image if composed slightly differently. But overall you did a very nice job both with your image and meeting the challenge.
HEE HEE - He seems so comfy and at ease posing for this shot. Certainly meets the challenge. Clear and well done. I'm a big fan of more negative space to one side of the subject, but that's just me.