Dermatology MCQ - Viral Infections - Mpox

A 35-year-old man presents with a prodrome of fever, lymphadenopathy, and headache, followed by the development of a generalized rash. The lesions began as macules, progressed to deep-seated, well-circumscribed vesicles and pustules, and are now umbilicated and crusting over. He reports the rash appeared first on his face and is spreading centrifugally to his extremities, including his palms and soles. He has no significant past medical history but reports recent intimate contact with a new male partner. Which of the following is the most appropriate initial diagnostic test?

A) Tzanck smear
B) Bacterial culture of pustule fluid
C) PCR testing of a vesicle swab
D) Darkfield microscopy
E) Viral culture

Correct Answer: C) PCR testing of a vesicle swab

Explanation

This presentation is highly characteristic of Mpox (formerly monkeypox), a zoonotic orthopoxvirus infection that can present with a clinical picture resembling smallpox but typically less severe.

Key Clinical Features of Mpox:

• Prodrome: Fever, chills, lymphadenopathy (a key feature distinguishing it from smallpox and chickenpox), headache, and myalgia.

• Rash Evolution: The rash evolves synchronously through stages: macules → papules → vesicles → pustules → crusts. The description of "deep-seated, well-circumscribed, and umbilicated" pustules is classic.

• Distribution: The rash often has a centrifugal distribution (concentrated on the face and extremities, including palms/soles) and can involve mucosal surfaces.

Diagnostic Confirmation:

• Polymerase Chain Reaction (PCR) Testing: This is the primary and most accurate diagnostic method. It detects mpox virus DNA.

  • Sample Collection: The optimal sample is swabbing the roof or base of a vesicle or pustule, or collecting crust material. Swabs should be placed in a sterile, dry container for viral transport media.

Why Not the Other Options?

• (A) Tzanck smear: This is a rapid test used primarily for herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections. It reveals multinucleated giant cells. It is not specific for mpox and would not provide a definitive diagnosis.

• (B) Bacterial culture: This is used to rule out a secondary bacterial infection (impetigo) of a skin lesion but will not identify the underlying viral cause.

• (D) Darkfield microscopy: This is used to identify the causative organism of syphilis (Treponema pallidum), which can cause a rash but not the vesiculopustular lesions described here.

• (E) Viral culture: While the mpox virus can be cultured, this is not the initial test due to longer turnaround times and the requirement for Biosafety Level 3 (BSL-3) facilities. PCR is faster, safer, and more specific.

Management:

• Supportive care is the mainstay for most cases.

• Antivirals like tecovirimat (Tpoxx) are considered for severe cases, immunocompromised patients, or those with complications.

• Infection Control: Patients should be advised to isolate until all crusts have separated and a fresh layer of skin has formed to prevent transmission.