Monarch Disease Ontology term MONDO_0023726 (mediastinal yolk sac tumor) is an extragonadal non-seminomatous malignant germ cell tumor that arises from the mediastinum. It is characterized by the presence of small pale cells with small amount of cytoplasm and round to oval nuclei with small nucleoli forming a variety of patterns, including microcystic, macrocystic, pseudopapillary, myxomatous, hepatoid, polyvesicular vitelline, and solid. It manifests with respiratory distress, thoracic pain, fever, and superior vena cava syndrome. Also known as: endodermal sinus neoplasm of mediastinum, endodermal sinus neoplasm of the mediastinum, endodermal sinus tumor of mediastinum, endodermal sinus tumor of the mediastinum, endodermal sinus tumour of mediastinum, endodermal sinus tumour of the mediastinum, mediastinal endodermal sinus neoplasm, mediastinal endodermal sinus tumor.