Answer: Echinococcal (hydatid) cyst.
The first image demonstrates the edge of the large cyst with collapsed laminated membranes, suggesting that this could be infection with Echinococcus granulosus. However, it is not possible to definitively determine the infecting species without seeing the entire lesion. E. granulosus generally causes a large cyst that can contain multiple daughter cysts. Each cyst/daughter cyst is surrounded by a laminated layer. In comparison, Echinococcus multilocularis forms multiple cysts separated from one another by only cellular stroma. Unlike E. granulosus, the cysts of E. multilocularis are not contained within a 'mother' cyst and often grow in an invasive fashion into the surrounding tissue.
The diagnosis of echinococcosis is made on the following features:
1. Characteristic laminated layer that is highlighted with GMS stain.
2. Presence of a single refractile hooklet
There were no other parasitic forms such as protoscoleces present in this case, and I had to look for about 30 minutes just to find the single hooklet (!) As noted by HLCM fan, the hooklets are best seen by closing the microscope's diaphragm.
The GMS stain is helpful for highlighting the laminations when no other parasitic forms are seen and differentiating the parasitic material from fibrin (which will not stain with GMS). GMS staining can be especially useful in cases of collapsed and degenerated cysts.
Thank you all for writing in on this case!
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