A 17-year-old man presents to the clinic with new symptoms of fatigue, malaise, fever, and a sore throat. He has no significant past medical history and is not on any medications. Physical examination is entirely normal except for enlarged, palpable cervical lymph nodes. He reports no weight loss or night sweats. Laboratory investigations include a normal chest x-ray, negative throat swab, but abnormal blood film with atypical lymphocytes. The hemoglobin is 15.5 g/dL; hematocrit 42%; platelets 290,000/mL; WBC 10500/mL, with 45% segmented neutrophils, 1% eosinophils, and 54% lymphocytes, of which 36% were atypical. Which of the following is the most appropriate initial diagnostic test?
A 17-year-old man presents to the clinic with new symptoms of fatigue, malaise, fever, and a sore throat. He has no significant past medical history and is not on any medications. Physical examination is entirely normal except for enlarged, palpable cervical lymph nodes. He reports no weight loss or night sweats. Laboratory investigations include a normal chest x-ray, negative throat swab, but abnormal blood film with atypical lymphocytes. The hemoglobin is 15.5 g/dL; hematocrit 42%; platelets 290,000/mL; WBC 10500/mL, with 45% segmented neutrophils, 1% eosinophils, and 54% lymphocytes, of which 36% were atypical. Which of the following is the most appropriate initial diagnostic test?
💡 Explanation
**Core Concept**
The patient's presentation with fever, sore throat, cervical lymphadenopathy, and atypical lymphocytes on the blood film is suggestive of infectious mononucleosis (IMN), also known as glandular fever. IMN is caused by the Epstein-Barr virus (EBV), a member of the herpesvirus family.
**Why the Correct Answer is Right**
The diagnosis of infectious mononucleosis can be confirmed by serological tests detecting the presence of specific antibodies against EBV. The most commonly used test is the Monospot test, which detects the presence of heterophile antibodies. These antibodies are produced in response to EBV infection and can be detected in the serum of most patients with IMN. The Monospot test is a rapid, point-of-care test that is widely available and can provide a presumptive diagnosis of IMN.
**Why Each Wrong Option is Incorrect**
**Option A:** While a complete blood count (CBC) with differential is an essential part of the diagnostic workup, it is not the most appropriate initial diagnostic test for infectious mononucleosis. The CBC will show atypical lymphocytes, but it is not specific for IMN.
**Option B:** A throat swab is not the most appropriate initial diagnostic test for infectious mononucleosis. While a throat swab can be used to diagnose bacterial pharyngitis, it is not typically used to diagnose viral infections like IMN.
**Option C:** A chest x-ray is not the most appropriate initial diagnostic test for infectious mononucleosis. While a chest x-ray can be used to diagnose pulmonary complications of IMN, such as pneumonia, it is not typically used as an initial diagnostic test.
**Option D:** A liver function test (LFT) is not the most appropriate initial diagnostic test for infectious mononucleosis. While LFTs can be used to diagnose liver dysfunction, which can occur in IMN, they are not specific for the diagnosis of IMN.
**Clinical Pearl / High-Yield Fact**
The Monospot test is a rapid, point-of-care test that can provide a presumptive diagnosis of infectious mononucleosis. However, a negative result does not rule out IMN, as some patients may not produce heterophile antibodies.
**Correct Answer:** B. Throat swab.
✓ Correct Answer: D. heterophil antibody (Monospot) test
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