Final answer:
The differential diagnosis for an abdominal mass in a 9-month-old includes abscess or pseudocyst in the pancreas, infection with schistosome parasites, appendicitis, UTI, or PID.
Step-by-step explanation:
The differential diagnosis (DDx) for an abdominal mass in a 9-month-old includes several conditions. One possible cause can be an abscess or pseudocyst in the pancreas, which can be detected using an abdominal ultrasound. Another possibility is an infection with schistosome parasites, which can cause liver and blood vessel damage and fluid accumulation in the abdominal cavity. Additionally, conditions such as appendicitis, urinary tract infection (UTI), or pelvic inflammatory disease (PID) can also cause abdominal masses, although they are typically accompanied by additional symptoms.