An 18-year-old woman presents to the clinic complaining of malaise for the past 3 weeks, fever for 2 weeks, and a sore throat. Physical examination shows pharyngeal infection with enlarged tonsils and a patchy, white exudate; enlarged, palpable anterior and posterior cervical, axillary, and inguinal lymph nodes; tenderness in the right upper quadrant; and minimal splenomegaly. Laboratory data show hemoglobin 14 g/dL; hematocrit 42%; platelets 380,000/mL; WBC 8500/mL, with 35% segmented neutrophils, 1% eosinophils, and 64% lymphocytes, of which 36% were atypical. Which of the following is the most likely diagnosis?

Okay, let's tackle this question. The patient is an 18-year-old woman with symptoms of malaise, fever, sore throat, and physical signs like pharyngeal infection with white exudate, enlarged lymph nodes in multiple areas, tenderness in the right upper quadrant, and splenomegaly. The lab results show a normal hemoglobin and platelet count but elevated WBC with a high percentage of lymphocytes, including 36% atypical ones.

First, the core concept here is likely infectious mononucleosis caused by Epstein-Barr virus (EBV). The symptoms and lab findings are classic. The atypical lymphocytes are a key indicator. Let's see why.

The correct answer is mononucleosis. The presence of fever, pharyngitis, lymphadenopathy, splenomegaly, and atypical lymphocytes in the blood is typical. The WBC count is elevated with a lymphocytic predominance, which is common in viral infections. EBV is the most common cause of this presentation.

Now, the distractors. Option A might be streptococcal pharyngitis, but that usually doesn't have the lymphadenopathy and atypical lymphocytes. Option B could be HIV, but the presentation is more acute here. Option C might be leukemias, but the clinical picture with EBV is more consistent. Option D could be another viral infection but EBV is the most classic.

Clinical pearl: Remember that EBV mono presents with the classic triad of fever, pharyngitis, and lymphadenopathy, plus atypical lymphocytes. Also, heterophil antibody test can help confirm.

**Core Concept**
This case centers on **infectious mononucleosis** caused by **Epstein-Barr virus (EBV)**, characterized by a distinct clinical and laboratory profile. Key features include **pharyngitis, lymphadenopathy, splenomegaly**, and **atypical lymphocytosis** (≥10% of lymphocytes), differentiating it from bacterial or other viral etiologies.

**Why the Correct Answer is Right**
EBV infection triggers a **T-cell-mediated immune response**, leading to **B-cell proliferation** and **atypical lymphocytes** in peripheral blood. The triad of **fever, pharyngitis, and lymphadenopathy**, coupled with **splenomegaly** and **atypical lymphocytosis** (36% here), strongly suggests mononucleosis. The **right upper quadrant tenderness** indicates splenic enlargement, a hallmark of EBV. White exudates on tonsils are common in EBV pharyngitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Streptococcal pharyngitis lacks lymphadenopathy, splenomegaly, and atypical lymphocytes. It presents with exudative pharyngitis, fever, and elevated neutrophils.
**Option B:** HIV seroconversion can mimic EBV but typically lacks the classic triad and has a more prolonged prodrome. CD4+ T-cell depletion occurs later.
**Option C:** Acute lymphoblastic leukemia (ALL) presents with **pancytopenia**, **leukocytosis** with blasts, and **hepatosplenomegaly**, not

✓ Correct Answer: C. infectious mononucleosis