Question: A 70-year-old woman presented to our hospital because of a 6-month history of melena. She had had a medical history of chronic hepatitis B and hepatitis C for decades. Moreover, she underwent a laparotomy 20 years previously. Physical examination revealed unremarkable abnormalities. Laboratory tests revealed a hemoglobin level of 9 g/dL. Esophagogastroduodenoscopy, colonoscopy, and small bowel series all failed to detect a bleeding source. Subsequently, spiral enteroscopy was performed perorally by using a specialized overtube (Endo-Ease Discovery Small Bowel; Spirus Medical, Inc., Stoughton, MA, USA) and an enteroscope (Fujinon EN-450T5; Fujinon Corp., Saitama, Japan). A giant subepithelial tumor with superficial ulceration, measuring 3-4 cm in diameter, was identified in the proximal jejunum (Fig. A). Spiral enteroscopy revealed a thickstalk tumor with a "cushion sign" by a grasping forceps (Fig. B). A gastrointestinal stromal tumor was suspected initially; however, the tumor revealed fat content with an attenuation coefficient of −98 Hounsfield units on CT (Fig. C. arrow). We decided to remove it endoscopically to prevent further risk of rebleeding or obstruction and applied an endoloop to ligate the stalk of the tumor (Fig. D). After looping of the tumor, a subtotal polypectomy using an electrosurgical snare was performed (Fig. E). The tumor showed a positive "naked fat sign" on the cutting surface after polypectomy (Fig. F). Pathological evaluation of the resected tumor revealed much adipose tissue with superficial ulceration (Fig. G. H&E, ×40). What is your diagnosis of this subepithelial tumor?