Question:medium
What is the investigation for an 8 yr old boy with a lesion on the back, featuring peripheral scaling and central scarring?
What is the investigation for an 8 yr old boy with a lesion on the back, featuring peripheral scaling and central scarring?
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Peripheral spread with central scarring points to lupus vulgaris (cutaneous TB), confirmed on skin biopsy.
Updated On: Jul 8, 2026
- Tzanck test
- KOH mount
- Skin biopsy
- Patch test
Show Solution
The Correct Option is C
Solution and Explanation
Step 1: Break down the clinical clue.
The lesion spreads outward at its edge (peripheral scaling) while the middle heals with a scar (central scarring). This growth pattern, slow expansion plus central healing, is more important than any single symptom here.
Step 2: Recognize the disease behind the pattern.
This is the textbook picture of lupus vulgaris, a long-standing form of skin tuberculosis. The disease grows very slowly over months to years and destroys tissue in the older, central part of the plaque while it is still actively spreading at the newer, outer edge.
Step 3: Pick the test that proves a granuloma.
Since lupus vulgaris is caused by tuberculous granulomas sitting in the skin, only a tissue sample can show this. A skin biopsy will reveal epithelioid granulomas with Langhans giant cells and often caseation, which confirms the diagnosis; special stains or PCR for TB can add extra confirmation from the same sample.
Step 4: Explain why the quicker bedside tests fail here.
A Tzanck smear only helps with blistering viral or autoimmune skin disease, and there are no blisters here. A KOH mount would only be useful if a fungal infection like tinea were suspected, and while tinea can also scale at the edges, it does not produce true central scarring, it typically shows central clearing instead. A patch test checks for contact allergy, which has nothing to do with an infectious granuloma.
Step 5: Conclude.
The only investigation that can actually confirm lupus vulgaris is tissue examination.
$\boxed{\text{Skin biopsy}}$
The lesion spreads outward at its edge (peripheral scaling) while the middle heals with a scar (central scarring). This growth pattern, slow expansion plus central healing, is more important than any single symptom here.
Step 2: Recognize the disease behind the pattern.
This is the textbook picture of lupus vulgaris, a long-standing form of skin tuberculosis. The disease grows very slowly over months to years and destroys tissue in the older, central part of the plaque while it is still actively spreading at the newer, outer edge.
Step 3: Pick the test that proves a granuloma.
Since lupus vulgaris is caused by tuberculous granulomas sitting in the skin, only a tissue sample can show this. A skin biopsy will reveal epithelioid granulomas with Langhans giant cells and often caseation, which confirms the diagnosis; special stains or PCR for TB can add extra confirmation from the same sample.
Step 4: Explain why the quicker bedside tests fail here.
A Tzanck smear only helps with blistering viral or autoimmune skin disease, and there are no blisters here. A KOH mount would only be useful if a fungal infection like tinea were suspected, and while tinea can also scale at the edges, it does not produce true central scarring, it typically shows central clearing instead. A patch test checks for contact allergy, which has nothing to do with an infectious granuloma.
Step 5: Conclude.
The only investigation that can actually confirm lupus vulgaris is tissue examination.
$\boxed{\text{Skin biopsy}}$
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