A. Jejunojejunal leak: This is the most likely diagnosis given the symptoms of abdominal pain, distension, and the presence of serous fluid in the drains. A leak from the jejunojejunal anastomosis can lead to these symptoms, especially if it occurs early in the post-operative period.

B. Duodenal blow out: Although less common than a jejunojejunal leak, a duodenal blowout (also known as a duodenal stump leak) is a possible complication after a total gastrectomy. It can present with similar symptoms, including abdominal pain and distension, and the presence of fluid in the drains.
C. Afferent loop syndrome: This condition occurs when there is an obstruction of the afferent limb of the small intestine after a gastric bypass or other surgical procedures involving the stomach. Symptoms can include abdominal pain, distension, and vomiting, which align with the patient's presentation.

Infection or Abscess: Although not explicitly listed among the options, infection or abscess formation at the surgical site or within the abdominal cavity is a critical diagnosis that must not be missed. The patient's fever (T 38.5) and abdominal tenderness suggest an infectious process, which could be related to any of the surgical complications listed.
Bleeding: Post-operative bleeding is another condition that must be considered, as it can lead to severe consequences if not promptly addressed. However, the presence of serous fluid rather than frank blood in the drains makes this less likely.

D. Post gastrectomy syndrome: This is a broader term that encompasses various symptoms that can occur after gastrectomy, including dumping syndrome, diarrhea, and malabsorption. While it's a possible long-term complication, it's less likely to present acutely in the immediate post-operative period with the symptoms described.
Other rare complications such as intestinal obstruction, pancreatitis, or cholecystitis could also be considered but are less directly related to the specific symptoms and surgical history provided.