A 50-year-old male patient presented with severe excruciating pain in the shoulder region along with swelling and fever. There was no history of any kind of trauma. Bullae filled with clear, cloudy, haemorrhagic, or purplish fluid were observed at the site of pain and the surrounding skin had a purple hue. A pleomorphic bacillus forming oval subterminal spores was observed on gram staining. It grew in anaerobic conditions on culture. The colonies were initially irregular and transparent which turned opaque on repeated incubations. The organism was saccharolytic and produced abundant gas on fuher studies. Which investigation should be carried out next to rule out the root cause of above condition: -