This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Medicine
What is the most likely underlying diagnosis in this 82-year-old patient with diabetes mellitus who had undergone a total hip replacement 10 years previously? 
Select and explain. 
A. Colon cancer, B. Hypogammaglobulinemia, C. Hypophosphatasia, D. Osteosarcoma
A: Colon cancer
Answer: A: Colon cancer 
Explanation: Answer A. Colon cancerThere is free air extending lateral to the greater trochanter. Intra-articular cultures grew Clostridium septicum, which has a known association with colorectal cancer. Subsequent colonoscopy revealed a fungating colonic adenocarcinoma.