This could just be a follicular cyst - many dogs have them come and go their whole lives. However, they usually aren't sensitive to the touch. My suggestion would be to take him in and have it looked at. If it's small enough, they can sometimes be removed w/ a quick biopsy punch. Otherwise, some sedation if they need to excise it. Once they send it to the lab, you'll know what it is.
Is it solid or mushy? Does it move? Does it have a whitehead? Is it red and irritated? If it's red, hot, painful, and has swelling around it -- then, you really need to suspect infection.
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°¨¨¨°ºOº°¨¨¨° Ann | Pfeiffer | Marcel Verdel Purcell | Wylie | Artie °¨¨¨°ºOº°¨¨¨° |