Correct Answer: B. HSV
HSV (Herpes Simplex Virus) is the classic causative organism of acute herpetic stomatitis and herpes labialis, presenting with painful vesicular lesions in the oral cavity and perioral region. The Tzanck smear is the pathognomonic diagnostic clue here—it reveals multinucleated giant cells (MGCs) with intranuclear inclusions, which are hallmark cytopathic effects of HSV infection. These MGCs form when infected epithelial cells fuse due to viral glycoprotein-mediated cell-to-cell fusion. The intranuclear inclusions (Cowdry type A inclusions) contain viral DNA and appear as homogeneous, eosinophilic masses that displace the nucleus peripherally. In Indian clinical practice, primary HSV-1 infection commonly presents in young adults with severe oral ulceration, fever, and regional lymphadenopathy (herpetic gingivostomatitis). The combination of facial pain, vesicular lesions, and the specific Tzanck smear findings makes HSV the definitive answer. Tzanck smear is a rapid, inexpensive bedside test widely used in Indian dermatology clinics for presumptive diagnosis before PCR or viral culture confirmation.
Why the other options are wrong
A. EBV — EBV causes infectious mononucleosis with pharyngitis and exudative tonsillitis, NOT vesicular lesions. While EBV-infected cells may show atypical lymphocytes on blood smear, Tzanck smear does not show multinucleated giant cells or intranuclear inclusions. EBV is a latent virus in B lymphocytes, not epithelial cells, so it does not produce the characteristic cytopathic effects seen here. C. Adenovirus — Adenovirus causes pharyngitis and conjunctivitis but does NOT produce vesicular lesions or the characteristic Tzanck smear findings. Adenoviral cytopathic effects include cell rounding and detachment, but NOT multinucleated giant cells with intranuclear inclusions. The clinical presentation of painful oral vesicles is atypical for adenovirus infection. D. Cytomegalovirus — CMV causes severe disease primarily in immunocompromised hosts (HIV/AIDS, transplant recipients) and presents with ulcers rather than vesicles. While CMV does produce intranuclear inclusions ('owl's eye' appearance), it does NOT typically cause multinucleated giant cells on Tzanck smear. CMV is rarely the cause of acute oral vesicular lesions in immunocompetent young adults.
High-Yield Facts
- Tzanck smear shows multinucleated giant cells and Cowdry type A intranuclear inclusions—pathognomonic for HSV (and VZV, but VZV causes dermatomal distribution).
- HSV-1 causes oral/facial herpes; HSV-2 causes genital herpes; both produce identical cytopathic effects on Tzanck smear.
- Primary herpetic stomatitis in young adults presents with severe oral ulcers, fever, and cervical lymphadenopathy; recurrent disease is milder and preceded by prodrome.
- Tzanck smear is rapid, inexpensive, and widely available in Indian clinics; sensitivity ~60–80% but highly specific for HSV/VZV when positive.
- Acyclovir is the DOC for HSV; topical acyclovir for mucocutaneous lesions, IV acyclovir for disseminated or CNS disease.
Mnemonics
TZANCK = HSV/VZV Signature Tzank smear → Zoster/HSV → Antigen/Antibody tests → Nucleated giant cells → Cowdry inclusions → Keratitis/stomatitis. When you see Tzanck smear in the question, think HSV or VZV immediately. MGC + Intranuclear = Herpes Family Multinucleated Giant Cells + Intranuclear Inclusions = Herpes (HSV or VZV). Use this when differentiating from other viral causes like adenovirus or EBV.
NBE Trap
NBE pairs oral vesicles with multiple viral causes (EBV, CMV, adenovirus) to test whether students know that Tzanck smear findings are specific to HSV and VZV. Students unfamiliar with Tzanck cytology may incorrectly choose EBV (which causes pharyngitis) or CMV (which causes ulcers in immunocompromised hosts).
Clinical Pearl
In Indian outpatient dermatology clinics, a young adult with painful oral vesicles and a positive Tzanck smear (MGCs + intranuclear inclusions) is presumptively diagnosed as HSV and started on acyclovir immediately, without waiting for PCR confirmation. This rapid diagnosis-to-treatment approach reduces morbidity and prevents secondary bacterial superinfection, especially in resource-limited settings.
_Reference: Robbins Ch. 8 (Viral Infections); Harrison Ch. 172 (Herpes Simplex Virus Infections); Jawetz Microbiology Ch. 37 (Herpesviruses)_