Answer: Giardia duodenalis (a.k.a. G. intestinalis, G. lamblia) trophozoites
This Giemsa-based stain nicely highlights all of the diagnostic features of Giardia trophozoites, including the flagella, which are not usually seen so well in trichrome or iron hematoxylin-stained stool preparations. The annotated image below shows some of the key diagnostic features:
Now to answer the important question that many of you asked - what are Giardia trophozoites doing in a lymph node?(!) Kudos to Anon, Florida Fan, and William Sears for correctly suggesting that the Giardia were a contaminant from the sampling method that was used.
What I didn't tell you is that the lymph node aspirate was performed using ultrasound-guided endoscopic biopsy. An endoscope was passed down the patient's esophagus, through their stomach and into the duodenum. Then, an aspirating needle was threaded through the endoscope and was used to take a sample of a neighboring lymph node by passing through the duodenal wall. Mystery solved!
This is the second case that I've seen of giardiasis detected incidentally during this procedure in the past 10 years, so it's important to understand how specimens are obtained.
Thank you for all of the great comments on this case. And thanks again to Dr. Heidi Lehrke for donating these beautiful pictures.