Infectious Mononucleosis (Mono): This condition, caused by Epstein-Barr virus (EBV), is characterized by prolonged fever, fatigue, and splenomegaly, which aligns with the patient's symptoms and ultrasound findings. The absence of other significant findings in the blood work and imaging also supports this diagnosis.

Tuberculosis (TB): Although the patient does not have typical symptoms of TB such as cough or weight loss, TB can present atypically, especially in younger individuals. The prolonged fever and splenomegaly could be indicative of abdominal or miliary TB.
Brucellosis: This zoonotic infection can cause prolonged fever, splenomegaly, and is often associated with occupational exposure to animals. The lack of specific symptoms does not rule out brucellosis, as it can have a varied presentation.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with prolonged fever and splenomegaly. Although less common in young adults without other symptoms, it remains a consideration given the splenomegaly.

Endocarditis: Infective endocarditis can present with prolonged fever and splenomegaly, especially if there is embolic phenomenon to the spleen. It's crucial to consider this diagnosis due to its high mortality rate if left untreated.
Sickle Cell Disease with Sepsis: Although the patient's CBC is reported as within normal limits, sickle cell crisis or sepsis in a patient with sickle cell disease could present with these symptoms. It's essential to rule out sickle cell disease, especially if the patient is from a high-prevalence area.
HIV Infection: Acute HIV infection can cause prolonged fever, and splenomegaly can be seen in various stages of HIV disease. Given the potential for severe consequences if not treated, HIV should be considered.

Castleman Disease: A rare lymphoproliferative disorder that can cause fever, splenomegaly, and lymphadenopathy. It's less likely but should be considered if other diagnoses are ruled out.
Sarcoidosis: Although more commonly associated with lung findings, sarcoidosis can cause splenomegaly and fever. It's a diagnosis of exclusion and might be considered if other tests are negative and there are other systemic symptoms.
Visceral Leishmaniasis: This parasitic infection can cause prolonged fever, splenomegaly, and is endemic in certain parts of the world. It would be considered based on travel history and exposure risk.