Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), it can lead to splenomegaly, and gastrointestinal symptoms like diarrhea and gastritis are common, especially in the acute phase.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can cause splenomegaly and may be associated with gastrointestinal symptoms due to lymphomatous involvement of the stomach or intestines.
Chronic Liver Disease: Conditions like cirrhosis can lead to splenomegaly due to portal hypertension. Diarrhea and gastritis can occur due to liver dysfunction or associated conditions like portal hypertensive gastropathy.
Sarcoidosis: This systemic granulomatous disease can cause splenomegaly and may involve the gastrointestinal tract, leading to symptoms like diarrhea and gastritis.

Tuberculosis (TB): Although less common in some regions, TB can cause splenomegaly and gastrointestinal symptoms, including diarrhea and gastritis, especially in immunocompromised patients. Missing this diagnosis can be fatal if not treated promptly.
HIV/AIDS: Patients with HIV/AIDS can present with splenomegaly and a variety of gastrointestinal symptoms due to opportunistic infections or the direct effect of the virus on the gastrointestinal tract and spleen.
Lymphatic or Gastrointestinal Cancer: Certain cancers, like lymphoma or gastric cancer, can cause splenomegaly and gastrointestinal symptoms. Early diagnosis is crucial for treatment and prognosis.

Whipple's Disease: A rare, systemic bacterial infection caused by Tropheryma whipplei, which can lead to splenomegaly, diarrhea, and gastritis, among other symptoms.
Mastocytosis: A condition characterized by the accumulation of mast cells in one or more organs, which can cause splenomegaly and gastrointestinal symptoms like diarrhea and gastritis due to mast cell degranulation.
Amyloidosis: A group of diseases characterized by the deposition of amyloid proteins in various organs, including the spleen and gastrointestinal tract, leading to splenomegaly and gastrointestinal symptoms.