Which stain is commonly used in the diagnosis of certain conditions, including villonodular synovitis?

The stain commonly used in the diagnosis of conditions such as villonodular synovitis is vimentin. Vimentin is an intermediate filament protein that is expressed in mesenchymal cells, which makes it a useful biomarker in identifying tumors or conditions that arise from connective tissues. In the context of villonodular synovitis, which is characterized by proliferation of synovial cells and mesenchymal tissue, vimentin helps to highlight the cells involved and assists in confirming the diagnosis pathologically.

Hematoxylin and eosin is a general staining method that provides a broad overview of tissue morphology but does not specifically identify mesenchymal origins. Cytokeratin is primarily utilized to identify epithelial cells and can be informative in carcinomas but is not suitable for diagnosing synovial conditions. Collagen stains, while useful for highlighting connective tissues, do not specifically indicate the presence of synovial cells or the type of proliferative process occurring in conditions like villonodular synovitis. Thus, vimentin stands out as the most relevant stain in this particular diagnostic context.