**Question:** An 18-year-old woman visits her physician because of 3 weeks of malaise, 2 weeks of fever, 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, with 35% segmented neutrophils, 1% eosinophils, and 64% lymphocytes, of which 36% were atypical. Which of the following is the most likely diagnosis?

A. Syphilis
B. Tonsillitis
C. Hepatitis
D. Malaria

**Correct Answer:** **B. Tonsillitis**

**Core Concept:**

The patient presents with symptoms and signs consistent with a viral or bacterial infection affecting the lymphatic system, tonsils, and liver, which is characterized by fever, malaise, and lymphadenopathy. The laboratory data demonstrate a leukocytosis with a predominance of lymphocytes and atypical lymphocytes.

**Why the Correct Answer is Right:**

The patient's symptoms, physical examination findings, and laboratory data strongly suggest a viral or bacterial infection affecting the tonsils and lymph nodes. Among the given options, tonsillitis is the most likely diagnosis.

**Why Other Options are Incorrect:**

A. Syphilis: This condition is caused by Treponema pallidum and presents with genital ulcers, skin rash, and neurological symptoms, not fever and lymphadenopathy.

C. Hepatitis: Hepatitis is caused by viral hepatitis and presents with jaundice, hepatomegaly, and elevated liver enzymes, not fever and lymphadenopathy.

D. Malaria: Malaria is caused by Plasmodium species and presents with fever, chills, and hemolytic anemia, not fever and lymphadenopathy. Additionally, the elevated platelet count (380,000/mm³) is not consistent with malaria.

**Clinical Pearls:**

1. Atypical lymphocytes are seen in conditions like infectious mononucleosis (mono), Epstein-Barr virus infections, and viral hepatitis. Atypical lymphocytes are not typically seen in malaria or syphilis.
2. The predominance of lymphocytes in the peripheral blood smear is more characteristic of viral infections like Epstein-Barr virus or herpes viruses.
3. Elevated platelet count is more consistent with conditions like dengue or viral hepatitis rather than malaria.

**Clinical Pearls:**

1. The patient's symptoms and signs, along with the presence of atypical lymphocytes and elevated platelet count, point towards viral infections such as infectious mononucleosis (Mono) or Epstein-Barr virus infections.
2. The elevated platelet count is more consistent with conditions like dengue or viral hepatitis, rather than malaria.