Enteric Fistula: The presence of a frothy discharge through the drain on the 3rd day post pancreatectomy or pancreatic necrosectomy is highly suggestive of an enteric fistula. This condition occurs when there is an abnormal connection between the intestine and the pancreatic duct or the drain site, allowing intestinal contents, including air and digestive enzymes, to leak out. The frothy nature of the discharge is indicative of the presence of bile or pancreatic enzymes mixed with air.

Pancreaticocutaneous Fistula: While similar to an enteric fistula, a pancreaticocutaneous fistula involves a direct connection between the pancreas and the skin, leading to the discharge of pancreatic secretions. The discharge may be frothy if it contains a significant amount of pancreatic enzymes and air.
Infection or Abscess: An infection or abscess at the surgical site could potentially cause a frothy discharge, especially if the infection involves gas-producing bacteria. However, the discharge would more likely be purulent and foul-smelling rather than frothy.

Necrotizing Pancreatitis with Infection: Although less likely to present with a frothy discharge, necrotizing pancreatitis complicated by infection could lead to gas in the tissues and potentially a frothy discharge if the infection involves the drain site. Missing this diagnosis could be catastrophic due to the high mortality rate associated with infected necrotizing pancreatitis.
Graft-Versus-Host Disease (GVHD) in Transplant Patients: In patients who have undergone a transplant, GVHD could potentially cause a variety of gastrointestinal symptoms, including diarrhea and potentially a frothy discharge if there is involvement of the pancreas or intestinal tract. However, this would be extremely rare and more relevant in the context of bone marrow or solid organ transplantation.

Pancreaticopleural Fistula: An extremely rare condition where there is a fistulous connection between the pancreas and the pleural space. If the fistula communicates with a drain, it could potentially cause a frothy discharge, but this would be highly unusual and more likely to present with respiratory symptoms.
Congenital Anomalies: In very rare cases, congenital anomalies of the pancreas or intestinal tract could potentially lead to abnormal connections that result in a frothy discharge post-surgery. However, these would typically be identified pre-operatively or during surgery.