What is the most likely diagnosis in a 23-year-old male with tender bilateral cervical lymphadenopathy, tonsillitis-like symptoms, lymphocyte-predominant leukocytosis (~17,000), splenomegaly, and mild transaminitis?

Infectious Mononucleosis (EBV Infection)

The most likely diagnosis is infectious mononucleosis caused by Epstein-Barr virus (EBV), given the classic triad of bilateral cervical lymphadenopathy, lymphocyte-predominant leukocytosis, and splenomegaly with transaminitis in a young adult.

Clinical Reasoning

This 23-year-old male presents with the pathognomonic features of EBV-induced infectious mononucleosis 1:

• Bilateral tender neck swelling (cervical lymphadenopathy) - characteristic of IM, unlike the typically unilateral presentation of bacterial lymphadenitis
• Tonsillitis-like pharyngitis - EBV is a frequent cause of acute pharyngitis accompanied by generalized lymphadenopathy and splenomegaly 1
• Lymphocyte-predominant leukocytosis (WBC 17,000) - atypical lymphocytes are the hallmark of IM
• Splenomegaly - present in the majority of IM cases
• Transaminitis - hepatic involvement with elevated liver enzymes is common in IM 2

The age (23 years) and presentation pattern strongly favor IM, as this is the typical demographic for symptomatic EBV infection in developed countries.

Key Distinguishing Features

Why NOT Other Diagnoses:

Streptococcal pharyngitis 1:

• Would NOT cause splenomegaly
• Would NOT cause transaminitis
• Typically unilateral lymphadenopathy
• Neutrophil-predominant, not lymphocyte-predominant leukocytosis

Kawasaki Disease 2:

• Requires ≥5 days of fever plus ≥4 of 5 principal criteria
• Cervical lymphadenopathy in KD is typically unilateral (not bilateral)
• Splenomegaly argues AGAINST KD - the presence of splenomegaly should prompt consideration of another diagnosis 2
• KD shows neutrophil predominance, not lymphocyte predominance
• More common in children <5 years old

Chronic Lymphocytic Leukemia 3:

• Requires absolute lymphocytosis >5,000/μL with characteristic mature-appearing lymphocytes
• Median age at diagnosis is 70 years
• Insidious onset without acute pharyngitis
• Not associated with tender lymphadenopathy

Nontuberculous Mycobacterial Lymphadenitis 4:

• Primarily affects children 1-5 years old
• Typically unilateral (95% of cases)
• NOT tender and rarely associated with systemic symptoms
• No splenomegaly or transaminitis

Diagnostic Confirmation

Immediate testing required:

• Heterophile antibody test (Monospot) - rapid screening test
• EBV-specific serology: VCA-IgM (acute infection marker), VCA-IgG, EBNA-IgG
• Complete blood count with differential - look for >10% atypical lymphocytes
• Liver function tests - document transaminitis severity
• Peripheral blood smear - identify atypical lymphocytes

Critical Management Points

Avoid amoxicillin/ampicillin - causes a characteristic maculopapular rash in 80-100% of IM patients (not a true allergy, but a drug-disease interaction).

Spleen precautions - advise against contact sports and heavy lifting for 3-4 weeks due to splenic rupture risk (occurs in 0.1-0.5% of cases, typically weeks 2-3 of illness).

Self-limited course - symptoms typically resolve within 2-4 weeks with supportive care (rest, hydration, NSAIDs for symptom relief).

Red flags requiring further workup:

• Symptoms persisting >4 weeks (consider chronic active EBV infection 5)
• Progressive lymphadenopathy despite clinical improvement
• Development of severe cytopenias
• Persistent high fevers beyond 2 weeks

The bilateral distribution of lymphadenopathy, lymphocyte predominance, and presence of splenomegaly with transaminitis make infectious mononucleosis the overwhelmingly most likely diagnosis in this clinical scenario 1, 6, 7.