Histologic (hematoxylin and eosin stain) findings of synchronous primary carcinoid tumor and primary adenocarcinoma arising within mature cystic teratoma of horseshoe kidney. (A) Adenocarcinoma component with infiltrating atypical glands. Original magnification ×200. (B) Adenocarcinoma component composed of cohesive pleomorphic epithelioid cells with adjacent necrosis. Original magnification ×100. (C) Adenocarcinoma component showing large cells with abundant eosinophilic cytoplasm, enlarged nuclei with contour irregularities, and occasional prominent nucleoli. Original magnification ×600. (D) Carcinoid tumor (left part of figure) and adenocarcinoma (right part of figure) components adjacent and closely apposed to each other with a clear transition zone. Original magnification ×100.