A 45 year old woman presented to the emergency department with an eight hour history of sudden onset abdominal pain. The pain was severe, sharp, and worse on movement. She felt nauseous but had not vomited. She had last opened her bowels earlier that day, passing a small amount of hard stool. Her medical history included osteogenesis imperfecta, which caused hip pain, and for which she took 140 mg of oxycodone daily. On examination she was in obvious distress. She was tachypnoeic at 22 breaths/min and tachycardic at 110 beats/min. Blood pressure, peripheral oxygen saturation, and temperature were all in the normal range. Her abdomen was exquisitely tender to palpation, with maximum tenderness in the right iliac fossa, localised guarding, and percussion tenderness. Digital rectal examination identified hard faeces in the rectum. Bowel sounds were absent. Initial blood tests showed haemoglobin 14.5 g/L (reference range 12.0-15.0), white cell count 17.5×109/L (4.0-11.0), C reactive protein 5.5 mg/L (0-8). Urea, electrolytes, and liver function tests were normal. A venous blood gas showed a raised lactate of 4.2 mmol/L (0.5-2.0; 1 mmol/L=9.01 mg/dL). Urgent chest radiography (in the erect position) was performed (fig 1⇓). ### 1 What abnormality is apparent on the erect chest radiograph? #### Short answer The erect chest radiograph shows a pneumoperitoneum with free gas under the diaphragm bilaterally. #### Long answer The erect chest radiograph shows crescentic collections of gas under the diaphragm bilaterally. The right hemidiaphragm is clearly delineated, with gas above and below (fig 2⇓). In addition, the gas indicated by the broken arrows may be under the central tendon of the diaphragm, but may also …