Infectious Mononucleosis: Key Clinical Features
Splenomegaly is the most accurate statement among the options provided, occurring in approximately 50% of patients with infectious mononucleosis, making it "more likely than not" to be present. 1, 2, 3
Evaluating Each Statement
Age and Gender Distribution
The statement that the patient is "likely an adolescent male" is partially correct but imprecise. Infectious mononucleosis most commonly affects adolescents and young adults aged 15-24 years 3, but there is no significant male predominance in the disease. The age range is accurate, but gender is not a distinguishing feature.
Splenomegaly Prevalence
Splenomegaly occurs in approximately 50% of patients with infectious mononucleosis 1, 2, 3. A recent meta-analysis found that the presence of splenomegaly has a positive likelihood ratio of 2.39 for diagnosing IM 4. This makes it "more likely than not" to be present, though it's important to note that:
- Splenomegaly may not be clinically apparent in all cases
- Abdominal ultrasound detects enlarged spleen in nearly 90% of cases when performed 5
- Splenic rupture, though rare (0.1-0.5%), is the most feared complication 3
Concurrent Streptococcal Infection
The statement that patients "cannot be infected with strep" is incorrect. While concurrent group A streptococcal pharyngitis is uncommon in IM, it can occur. A study of 100 consecutive IM patients found only 4% had positive throat cultures for group A beta-hemolytic streptococci 6. The IDSA guidelines explicitly note that Epstein-Barr virus pharyngitis and streptococcal pharyngitis can overlap, though the signs and symptoms overlap so broadly that clinical diagnosis alone is unreliable 1, 2.
Clinical pitfall: The similarity between IM and streptococcal pharyngitis leads to inappropriate antibiotic use—one study found 82.6% of IM patients received antibiotics in the prehospital period 5.
Cervical Lymphadenopathy
Cervical lymphadenopathy is indeed prominent in infectious mononucleosis, being part of the classic triad of fever, tonsillar pharyngitis, and lymphadenopathy 3. The IDSA guidelines describe it as "generalized lymphadenopathy" that accompanies EBV pharyngitis 1, 2. Posterior cervical lymphadenopathy has a positive likelihood ratio of 3.16 for diagnosing IM 4.
Additional Key Clinical Features
Beyond the statements in question, clinicians should recognize:
- Periorbital/palpebral edema occurs in one-third of patients and is more common in those with splenomegaly 3, 7
- Hepatomegaly occurs in approximately 10% of cases 3
- Laboratory findings: Lymphocytosis with >50% lymphocytes and >10% atypical lymphocytes strongly suggests IM 3, 4
- Abnormal liver chemistries occur in 90% of patients 8
Diagnostic Approach
When IM is suspected:
- Perform heterophile antibody testing (monospot test) for rapid diagnosis 3
- If monospot is negative but clinical suspicion remains high, obtain EBV-specific serology (viral capsid antigen antibodies) 3, 9
- Do not empirically treat with antibiotics based on pharyngitis alone—only treat if throat culture confirms streptococcal infection 6, 5
- Avoid amoxicillin/ampicillin as they cause rash in 90% of IM patients 5