My three year old son has a rash on his elbows, lower back and on his buttocks, front and back of his legs. He has complained of leg pain in one leg and it s been around his calf muscle. There has been no fever and his diet and sleep habits have been normal.
An image of the rash is very helpful in diagnosis, but putting all your description together : rash on lower back, lower limbs and arthralgia urge me to ask: is this skin rash palpable ? Can you feel it ? If put your fingers around a spot and move your fingers from each other, does it blanch or go away ? Any previous cold or a runny nose in the past 2 weeks? If this rash doesn't blanch, and you can feel it, if it was associated with abdominal pain then there is a chance that your child has HSP (Enoch schooling purpura) and you should refer him to his pediatrician to order some tests. I hope this helps