What do you call a sore on a police officer's foot?

A sore on a police officer's foot is called Policeman's Foot Syndrome.

Explanation:

Policeman's Foot Syndrome is caused because of the long hours spent working (or doing any activity) on feet. This exigency leads to feeling pain in the area under the heels. It can affect any person, but it took its name from the times in which officers patrolled suburbs on foot. Its scientific name is plantar fasciitis.

Answer: Corn on the cop.

The joke relies on the comparison to the phrase "corn on the cob," and the use of "corn" to define both the crop and a foot injury.