What is this fruit?

This looks like a lemon crossed with a pear or apple. The fruit is the size of a lemon with a bumpy skin. I have never seen one before.

[Sue Cooper]

Ooh, that’s interesting. I wonder what it is. I challenge you to find out, Rog.

[xbm]

Looking on the Internet, Lis thinks it is a guava. I have never seen these before.