A 15-year-old boy presents with a 7-day history of a sore throat, low-grade fever, and malaise. He states that a week ago he had a fine rash over his body that faded quickly. On examination, you note the presence of mild splenomegaly. He is eating and drinking well, and his observations are normal. A heterophile antibody test is positive.

1. What is the most likely diagnosis?

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This patient has a diagnosis of infectious mononucleosis. Clinical infection is most common in populations with large numbers of young adults, such as university students and active-duty military personnel.

The main clinical features of infectious mononucleosis are:

  • Low-grade fever
  • Fatigue and prolonged malaise
  • Sore throat (tonsillar enlargement and exudate are common)
  • Transient, fine, macular, non-pruritic rash
  • Lymphadenopathy (most commonly cervical)
  • Arthralgia and myalgia
  • Mild hepatomegaly and splenomegaly
  • Jaundice (<10% if young adults but more common in the elderly)

2. What is the most common causative organism?

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Infectious mononucleosis is most commonly caused by the Epstein-Barr virus (EBV), which belongs to the human herpes virus family. Approximately 10% of cases are caused by cytomegalovirus (CMV) infection.

3. How is this condition usually treated?

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Treatment is generally symptomatic and includes the following:

  • Maintain good hydration status
  • Manage fever and myalgia with paracetamol and/or NSAIDs
  • 2% lidocaine (Xylocaine) solution as spray or gargle for sore throat


Current advice is that exclusion from school, and other childcare settings are not necessary for affected children. (unless it is appropriate for their own wellbeing).

Vigorous activities, such as contact sports, should be avoided for at least three to four weeks due to the risk of splenic rupture. Most cases of splenomegaly will resolve within this time period, but if there is any doubt ultrasound findings can guide management.

Ampicillin and amoxicillin should not be used in patients with infectious mononucleosis infection as they cause a rash. A diagnosis of infectious mononucleosis should be considered in children or teenagers with sore throats and fatigue or myalgia.

4. When might hospital admission be necessary?

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Urgent hospital admission may be required in patients with:

  • Severe or complicated disease
  • Enlarged tonsils causing airway obstruction
  • Poor fluid intake



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