Answer: D - Gastric glomus tumor

Glomus tumor is a mesenchymal neoplasm consisting of modified smooth muscle cells that resemble perivascular glomus body cells. Although these tumors are usually found in the distal extremities, they also rarely occur in visceral organs, with the stomach being the most frequent location.

Gastric glomus tumors (GGT) present as well-delineated intramural masses, centered in the muscularis propria. Microscopically, the tumor is composed of solid nodules that are often separated by bundles of muscularis propria. Tumor cells consist of a monomorphic population of round cells with sharply-defined cell membranes, round nuclei with delicate chromatin, and modest amounts of clear-to-eosinophilic cytoplasm. Most tumors are benign with rare mitotic activity, and can be cured by surgical resection. Rarely, however, some may have malignant histology with marked nuclear atypia and/or atypical mitotic figures. 

Immunohistochemically, GGTs can be distinguished from other neoplasms in the differential diagnosis by their expression of smooth muscle actin (SMA). A potential diagnostic pitfall is that GGT can be mistaken for carcinoid tumor, given their apparent insular growth pattern, monomorphic cell population, and presence of of synaptophysin immunoreactivity. Unlike carinoid tumor, however, GGT lacks cytokeratin expression and its epicenter is within the muscularis propria (as opposed to the deep mucosa/submucosa for carcinoid tumors). Compared to epithelioid GIST, GGT lacks immunoreactivity for CD117, as well as DOG1 in the majority of cases. Finally, GGTs do not have the S100 expression, malignant cytology, and occasional pseudopapillary structures characteristic of GI tract clear cell sarcoma-like tumor.