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DR8.1-5 | Common Viral Infections — Practice Quiz
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A 22-year-old woman presents with a cluster of small, tense vesicles on an erythematous base at the right corner of her mouth. She says the same area 'tingles' for a day before each episode and has had 3 similar episodes in the past year. Which virus is responsible?
Correct. HSV-1 establishes latency in the trigeminal ganglion and reactivates to cause recurrent orolabial herpes. The tingling prodrome plus grouped vesicles on an erythematous base at a fixed perioral site is classic.
HSV-1 causes orolabial (perioral) herpes; HSV-2 is the predominant cause of genital herpes. Recurrence at the same site after a tingling prodrome is the hallmark of herpes simplex.
Recurrent grouped perioral vesicles with a prodrome point to HSV-1, which preferentially causes orolabial disease. HSV-2 mainly causes genital herpes; VZV causes a dermatomal zoster or widespread varicella, not recurrent focal perioral lesions; MCV produces solid umbilicated papules, not vesicles.
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A Tzanck smear is performed on a fresh vesicle from a patient with a dermatomal eruption. The smear shows multinucleate giant cells. Which of the following conclusions is correct?
Correct. Multinucleate giant cells on Tzanck smear are the cytopathic effect common to all herpesviruses. While the dermatomal distribution strongly suggests VZV (zoster), the Tzanck smear itself cannot distinguish HSV from VZV — that distinction requires PCR or immunofluorescence.
Tzanck smear showing multinucleate giant cells is positive for herpesviruses (HSV and VZV) but cannot distinguish HSV from VZV. It is not positive in molluscum or HPV warts.
Tzanck smear shows multinucleate giant cells in BOTH HSV and VZV infections — it confirms a herpesvirus, not a specific species. MCV shows Henderson-Paterson (molluscum) bodies on histology; HPV warts show koilocytes, not giant cells.
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A 5-year-old child presents with fever followed by an intensely itchy rash. On examination the rash is most dense on the trunk and scalp, with sparse involvement of the limbs. You observe macules, papules, vesicles, and crusts all present simultaneously. The most likely diagnosis is:
Correct. The centripetal distribution and polymorphism (all stages simultaneously) are the two defining recognition features of varicella. Herpes zoster is dermatomal and does not generalise in immunocompetent children.
Varicella is recognised by its centripetal distribution (trunk/scalp denser than extremities) and simultaneous presence of lesions in multiple stages (polymorphic rash) — macules, papules, vesicles, and crusts at the same time.
The key clues are centripetal spread and polymorphism. Herpes zoster stays within a single dermatome in immunocompetent individuals. Disseminated HSV is rare and typically in immunocompromised hosts. Impetigo produces honey-crusted superficial lesions without preceding vesicles at multiple stages.
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A 55-year-old man presents with severe pain over the left thorax for 3 days, followed by a band of grouped vesicles on an erythematous base that stops exactly at the midline. He has no prior similar episodes. The most appropriate antiviral to initiate is:
Correct. Aciclovir (or valaciclovir, which is better absorbed) is the standard antiviral for herpes zoster. Early initiation reduces acute pain duration and the incidence of post-herpetic neuralgia. Oseltamivir targets influenza neuraminidase; tenofovir is an antiretroviral; cryotherapy is not used for zoster.
Herpes zoster (VZV reactivation) is treated with aciclovir or its prodrug valaciclovir to shorten the acute episode and reduce the risk of postherpetic neuralgia (PHN). Early treatment (within 72 hours of rash onset) gives the greatest benefit.
The unilateral dermatomal vesicular eruption stopping at the midline is herpes zoster (VZV reactivation). Aciclovir/valaciclovir is the correct antiviral. Oseltamivir is for influenza; tenofovir is an antiretroviral; zoster always requires antiviral management, especially in those >50 to prevent PHN.
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A 30-year-old man presents with a painful lesion on the sole of his right foot. Examination shows a hyperkeratotic plaque that hurts when pressed directly. You pare the lesion and find that the normal skin-line pattern (dermatoglyphics) is interrupted, and tiny black dots are visible in the centre. The most likely diagnosis is:
Correct. Interrupted skin lines plus black dots (thrombosed capillaries) after paring = plantar wart. A corn allows the skin lines to run through and has no black dots. This paring test is the essential bedside discriminator.
The single most useful bedside test to distinguish a plantar wart from a corn is paring: a plantar wart interrupts the skin lines and shows central black dots (thrombosed capillaries), whereas a corn preserves the skin lines and has no black dots. Pain pattern also differs (wart = direct pressure; corn = lateral pressure).
The key finding after paring is interrupted skin lines + black dots = plantar wart. A corn preserves the skin line pattern and has a translucent central core. Molluscum presents with pearly umbilicated papules, not a plantar hyperkeratotic plaque. Tinea pedis causes scaling, maceration, and blistering — not a discrete hyperkeratotic plaque.
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Which of the following correctly describes the causative organism of molluscum contagiosum?
Correct. MCV is a poxvirus; like all poxviruses it replicates in the cytoplasm (not the nucleus), producing the characteristic eosinophilic Henderson-Paterson (molluscum) bodies seen on histology.
Molluscum contagiosum virus (MCV) is a poxvirus — the only large DNA virus that replicates entirely in the cytoplasm. This cytoplasmic replication produces the histological hallmark: Henderson-Paterson (molluscum) bodies within keratinocytes.
MCV is a poxvirus with cytoplasmic replication — unique among DNA viruses. Herpesviruses replicate in the nucleus and establish ganglion latency. Papillomaviruses (HPV) replicate episomally in keratinocyte nuclei. Paramyxoviruses are RNA viruses (not relevant here).
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A 7-year-old girl presents with multiple 2–4 mm pearly, dome-shaped, flesh-coloured papules on her trunk. On close inspection each papule has a small central dimple. There is no pain or itching. The most likely diagnosis is:
Correct. The central umbilication (dimple) is the diagnostic fingerprint of molluscum contagiosum. Viral warts are hyperkeratotic and lack umbilication; varicella is vesicular; milia are tiny white epidermal cysts without an umbilicated dimple.
The central umbilication (dimple) on a pearly dome-shaped papule is the pathognomonic feature of molluscum contagiosum. Multiple lesions on the trunk in a school-age child is the classic epidemiological setting.
The central umbilication is the key distinguishing sign. Viral warts are rough, hyperkeratotic lesions without umbilication. Early varicella vesicles have an erythematous base and progress through stages. Milia are tiny (1–2 mm) whitish keratin cysts without umbilication and do not appear in clusters on the trunk in a school-age child.
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Which statement about postherpetic neuralgia (PHN) following herpes zoster is correct?
Correct. PHN is defined as pain lasting >3 months after acute zoster and is substantially more common in patients over 60. Early aciclovir/valaciclovir reduces but does not eliminate the risk. The recombinant zoster vaccine (Shingrix) further reduces PHN incidence.
PHN is defined as pain persisting more than 3 months after the acute zoster rash. Its incidence rises sharply with age (>60 years). Early antiviral therapy (within 72 hours) and the herpes zoster vaccine both reduce the risk of PHN.
PHN is not transient — it persists >3 months. It is a disease of the elderly (rises sharply after 60). Early antivirals DO reduce PHN risk, which is one of the main reasons to treat zoster promptly in older patients.
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A 19-year-old college student develops multiple plane warts (verruca plana) on his face. Compared with common warts (verruca vulgaris), plane warts characteristically:
Correct. Plane warts are flat-topped, smooth, and skin-coloured or slightly brownish. They frequently display Koebner phenomenon — linear auto-inoculation along scratch lines on the face or dorsal hands. This contrasts with common warts' rough, dome-shaped, heavily keratinised surface.
Plane warts (verruca plana) are flat-topped, flesh-coloured or light-brown, minimally keratinised papules that may be distributed in linear arrays due to auto-inoculation along scratch lines (Koebnerisation). They differ from common warts, which are dome-shaped and heavily keratinised.
Plane warts are characterised by their flat top and smooth surface — they are NOT exophytic or heavily keratinised. The linear Koebner pattern from scratching is a distinguishing clue. Spontaneous resolution can occur over months to years but not within 2 weeks.
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