Q: My newborn son has developed a hard yellow rash on his bum. Any ideas on what this could be? Could it be an allergic reaction to petroleum jelly, or some sort of infection?
A: I have no idea what this rash is based on the written description. Sounds like caked on diaper rash cream maybe? But you will need to have your baby seen by his doc to take a look and figure it out if a bath and some gentle cleansing doesn't remove it.