A 72-year-old man presents to his GP with increasing dyspepsia and weight loss. He has not experienced any other GI symptoms and physical examination is unremarkable. A barium meal is performed with the administration of intravenous Buscopan. The oesophagus is normal in appearance but a 'bull&;s eye' lesion is noted in the gastric mucosa. Which one of the following is not a recognised cause of this appearance?