This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': 'The following occlusal radiograph shows: \nSelect and explain. \nA. Submandibular calculus, B. Parotid  calculus, C. Sublingual calculus, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Intraoral Radiography\nSialoliths in the anterior two-thirds of the submandibular duct are typically imaged with a cross-sectional mandibular occlusal projection. The posterior portion of the duct may be demonstrated with an over-the-shoulder occlusal projection\xa0view,\xa0where\xa0the\xa0directing\xa0cone\xa0is\xa0placed\xa0on\xa0the shoulder\xa0and\xa0the\xa0central\xa0ray\xa0is\xa0directed\xa0in\xa0an\xa0anterior\xa0direction through the angle of the mandible, with the patient’s head rotated back and tilted to the unaffected side.\nParotid\xa0sialoliths\xa0are\xa0more\xa0difficult to\xa0demonstrate\xa0than\xa0the submandibular variety owing to the tortuous course of Stensen’s duct around the anterior border of the masseter and through the buccinator muscle.\xa0\nAs a rule, only sialoliths anterior to the masseter muscle can be imaged on an intraoral image. To demonstrate sialoliths in the anterior part of the duct, an intraoral image receptor is stabilized with a holder inside the cheek, as high as possible in the buccal sulcus and over the parotid papilla. The central ray is directed perpendicular to the center of the receptor.\nKey Concept', 'role': 'assistant'}]