Answer: Schistosoma mansoni eggs
This case seemed to generate a lot of enthusiasm from the audience. Thank you all for the great comments! The eggs shown here are classic for S. mansoni, with their large size (>150 microns largest dimension) and classic lateral spine.
Chris C. asked how I thought to do the squash preparation on this tissue. The answer is that this tissue was submitted specifically to look for Schistosoma eggs. As you can imagine, this is a rare request when you practice in an area that does not have endemic schistosomiasis. However, we do on occasion receive rectal biopsies in microbiology for this technique. It's important that the tissue be sent fresh, since fixed tissue will not squash easily, and does not provide a good preparation.
If tissue has already been placed in formalin, then it is best to submit it to surgical pathology. Unfortunately, you then get a lot of fixation artifacts, and the eggs become very distorted, as seen below:
By looking carefully, you still may be able to find a lateral spine. But be careful about calling 'spines' when all you are seeing are collapsed, angulated eggs. You'll want to be sure that you are seeing a well defined spine in the expected position. Here is a really good example of a lateral spine that survived fixation, dehydration, and sectioning: