<prompt> Which of the following immunohistochemical marker is positive in neuroendocrine tumor?CytokeratinSynaptophysinCalretininGEAP  <response>: Ref. Robbins Pathology. 9th edition. Page. 1099Though Cytokeratin is also positive in neruroendocrine carcinoma. For neuroendocrine differentiation more specific marker is synaptophysin and chromogranin WHO classificationTypical carcinoids tcsAtypical carcinoids acsSmall cell carcinomas sccsLarge cell necs LCNECs Carcinoid grade I ruled out because it lacks comedo-like necrosis, has 0.2 mitotic figuresHP Atypical carcinoid:Increased mitotic activity - per 2-1010 HPFPleomorphism and irregular nuclei with hyperchromatism and prominent nucleoliAreas of increased cellularity with loss of the regular, organized architecture observed in typical carcinoidAreas of necrosis within the tumor Small cell carcinoma has extensive necrosis not comedotype as shown in the picture, mitotic figure usually very high 2010hpf and weak staining by neuroendocrine markers IHC - Neuroendocrine specific Synaptophysin, chromogranin, CD56 