Viral Infections (e.g., Mononucleosis): This is often the most common cause of mild splenomegaly in young adults, due to the high prevalence of infectious mononucleosis (caused by Epstein-Barr virus) in this age group. The infection can lead to spleen enlargement, which is usually mild and self-limiting.

Chronic Liver Disease: Conditions like chronic hepatitis B or C can lead to portal hypertension, which in turn can cause splenomegaly. Although less common in young adults without a history of liver disease, it's a consideration, especially with risk factors for liver disease.
Lymphoma: Although more commonly associated with significant lymphadenopathy, some forms of lymphoma can present with splenomegaly. Given the age group, Hodgkin lymphoma could be a consideration.
Autoimmune Disorders (e.g., Systemic Lupus Erythematosus): These conditions can cause splenomegaly due to the body's immune response. SLE, for example, can affect multiple organs, including the spleen.

Sickle Cell Disease: While often diagnosed in childhood, some cases might not be identified until later. Splenomegaly can occur due to sickling of red blood cells within the spleen, leading to its enlargement.
Leukemia: Acute or chronic leukemias can cause splenomegaly. Given the potential for rapid progression and the importance of early treatment, leukemia is a critical diagnosis not to miss.
Lymphatic or Venous Obstruction: Conditions that obstruct the flow of blood or lymph from the spleen can lead to its enlargement. Examples include splenic vein thrombosis or lymphoma obstructing lymphatic vessels.

Gaucher's Disease: A genetic disorder that leads to the accumulation of glucocerebroside in cells, which can cause splenomegaly among other symptoms. It's rare but should be considered in cases where common causes are ruled out.
Splenic Cysts or Abscesses: These can cause localized enlargement of the spleen and are usually identified through imaging studies.
Histiocytosis: A group of diseases involving abnormal proliferation of histiocytes, which can lead to splenomegaly. It's rare and often presents with other systemic symptoms.
Amyloidosis: The deposition of amyloid proteins in the spleen can cause its enlargement. This condition is rare and often associated with other systemic symptoms or diseases.