Most of dermatology is vocabulary, distribution, and one question about sensation.
Skin disease is intimidating largely because it looks like an infinite catalogue. It is much more tractable than that. Learn a small vocabulary for describing lesions, pay attention to where the rash is rather than only what it looks like, and a large proportion of common conditions declare themselves. Flexures suggest eczema, extensor surfaces suggest psoriasis, finger web spaces with a night-time itch suggest scabies.
Two things then sit on top of that. A small number of rashes are emergencies, and one condition — leprosy — is found by a test that takes seconds and is routinely omitted.
🩺 Where this lives: The disability caused by leprosy comes from nerve damage, not from the organism — and by the time deformity appears, the damage is done. A patient with anaesthetic hands burns and cuts them without noticing; the wounds become infected, tissue is lost, and the visible deformity that carries such profound stigma follows. Every stage of that is preventable by early diagnosis, and early diagnosis usually depends on one simple act: testing sensation in a hypopigmented patch. Leprosy remains endemic in parts of Nepal, and the patient will not volunteer that the patch is numb, because they have not noticed.
💡 A note on treatment. This chapter gives no doses and no leprosy regimen — multidrug therapy is classification-dependent and specified by the national leprosy programme and WHO guidance. Childhood rash illnesses are covered in the Childhood Infections chapter, necrotising infection in the Wounds and Surgical Infection chapter, and allergy versus intolerance in the Prescribing Safety chapter.
Describing a rash
Using the right words is not pedantry — it is how you get useful advice from a colleague you are telephoning, and it is what examiners are testing. Beyond the individual lesion, always record the distribution (where, and is it symmetrical?), the surface (scaly, crusted, weeping?), and whether it itches. And in any acutely unwell patient, apply the glass test: whether a rash blanches changes everything.
Eczema, psoriasis and scabies
THE DISTRIBUTIONS THAT DO THE WORK
ECZEMA
Itchy, ill-defined, dry, scaly. In children classically
the FLEXURES — behind the knees, in front of the elbows.
Often a personal or family history of atopy: asthma, hay
fever. Weeping and crusting suggest secondary infection.
PSORIASIS
Well-DEMARCATED plaques with SILVERY scale, classically
on EXTENSOR surfaces — elbows and knees — plus scalp and
nails. Nail pitting and onycholysis support it. May be
accompanied by an inflammatory ARTHRITIS.
SCABIES
Intense itch, characteristically WORSE AT NIGHT. Look in
the finger WEB SPACES, wrists, axillae and genitalia.
THE DECISIVE QUESTION: IS ANYONE ELSE IN THE HOUSE
ITCHING? Scabies is transmitted by prolonged close
contact, so it clusters in households.
AND THE COMMONEST TREATMENT FAILURE is not treating
everyone in the household simultaneously, and not
treating clothing and bedding — the patient is cured and
then promptly reinfected. Itch may persist for some weeks
after successful treatment, which is not failure.
A PRACTICAL POINT: eczema and psoriasis both improve with
emollients and worsen when the skin dries out. Simple
measures matter more than they sound.
Leprosy
💡 Exam angle: the cardinal signs are worth knowing exactly — a skin patch with definite loss of sensation, thickened peripheral nerves, and a positive skin smear. The examinable behaviour is even simpler: test sensation in every hypopigmented patch. Most such patches are something else entirely, but the test costs seconds and is what separates a curable early case from a lifetime of deformity. Also palpate the peripheral nerves for thickening — it is a physical sign students rarely practise and examiners like.
Skin infections
Two patterns are worth having ready. Fungal infection gives an annular lesion with an active raised scaly edge and central clearing — the ring is the point. And herpes zoster gives painful vesicles confined to a single dermatome that do not cross the midline, often with pain preceding the rash by days. The important safety note in cellulitis is the one from the surgical chapter: pain out of proportion to the appearance suggests necrotising infection, which is a surgical emergency rather than an antibiotic problem.
Dermatological emergencies
THE RASHES THAT CANNOT WAIT
NON-BLANCHING PURPURIC RASH WITH FEVER
Meningococcal disease until proven otherwise. Give
ANTIBIOTICS IMMEDIATELY, before lumbar puncture, imaging
or transfer. Covered fully in the Childhood Infections
chapter — and it applies to adults too.
SEVERE DRUG REACTION
The features that mark a rash as dangerous rather than
trivial:
MUCOSAL INVOLVEMENT — mouth, eyes, genitals
BLISTERING or skin peeling
Systemic illness, fever
Target lesions
Skin pain or tenderness
STOP THE SUSPECT DRUG IMMEDIATELY and get specialist
help. These reactions carry significant mortality and
are managed like burns, with attention to fluid loss,
infection and the eyes.
A drug rash typically appears days to weeks after
starting the drug — so ask about everything started in
the preceding weeks, including antibiotics and
antiepileptics.
ERYTHRODERMA — widespread redness affecting most of the
skin surface. The skin fails as an organ: fluid loss,
heat loss, protein loss and infection risk. It needs
admission.
ANGIO-OEDEMA involving the airway is an anaphylaxis
problem rather than a rash — treat accordingly.
Clinical reasoning: four presentations
🔍 Case 1 — a pale patch on the arm
PresentationA 24-year-old has a single hypopigmented patch on his forearm, present for months. It is not itchy. He is given an antifungal cream and told to return if it does not clear.
The test not doneSensation.
ReasoningA hypopigmented patch that is not itchy is not typical of fungal infection, and definite loss of sensation within a patch is a cardinal sign of leprosy. The patient will not report numbness because he has not noticed it.
AnswerTest sensation within the patch and palpate the peripheral nerves for thickening. If abnormal, refer per the national leprosy programme — early treatment prevents nerve damage and deformity.
🔍 Case 2 — the itch that keeps coming back
PresentationA woman treated for scabies twice returns with the same intense night-time itch and burrows in the finger webs. Her husband and two children are also scratching, and none has been treated.
Why treatment failedOnly the patient was treated.
ReasoningScabies clusters in households through prolonged close contact. Treating one person while untreated contacts remain guarantees reinfection.
AnswerTreat all household contacts simultaneously, whether or not they itch, and address clothing and bedding per protocol. Warn that itch may persist for weeks after successful treatment.
🔍 Case 3 — a rash with a sore mouth
PresentationA patient started on a new drug ten days ago develops a widespread rash with painful mouth ulcers, red eyes and areas of blistering. He is febrile. He is given an antihistamine and advised to continue his medication.
Red flagsMucosal involvement, blistering, systemic illness.
ReasoningThis is a severe cutaneous drug reaction, not a simple rash. Mucosal involvement with blistering marks a condition carrying significant mortality, and continuing the drug worsens it.
AnswerStop the suspect drug immediately, admit, and obtain urgent specialist input. Manage fluid loss, infection risk and eye involvement per protocol.
🔍 Case 4 — cellulitis that hurts too much
PresentationA diabetic man is treated for lower leg cellulitis. The pain is far more severe than the modest redness suggests, he is systemically unwell, and there is now an area of dusky discolouration. Antibiotics are continued.
The discriminatorPain out of proportion to the appearance.
ReasoningNecrotising soft tissue infection spreads in the fascial plane beneath the skin, so the surface can look unremarkable while the patient is severely ill. It is a surgical emergency.
AnswerUrgent surgical review for debridement, alongside resuscitation and broad-spectrum antibiotics — see the Wounds and Surgical Infection chapter.
Commonly confused
Confusion
The distinction
Why it matters
Eczema vs psoriasis
Flexures and ill-defined versus extensors and demarcated
Distribution usually settles it.
Fungal patch vs leprosy patch
Test the sensation; fungal itches and has an active edge
Misses the diagnosis that causes disability.
Treating the patient vs the household
Scabies clusters in households
The commonest cause of treatment failure.
Persistent itch vs treatment failure
Itch can persist weeks after cure
Prevents unnecessary retreatment.
Simple vs severe drug rash
MUCOSAL involvement and blistering
One is stopped and observed; one is an emergency.
Blanching vs non-blanching
The glass test
Fever plus non-blanching means antibiotics now.
Cellulitis vs necrotising infection
Pain out of proportion, toxicity
One needs antibiotics, one needs theatre.
Zoster vs other vesicular rash
Single dermatome, not crossing the midline
A clean anatomical discriminator.
Rapid revision
MUST-KNOW FACTS
1. MACULE flat and small · PATCH flat and large.
2. PAPULE raised and small · PLAQUE raised and large · NODULE deep.
3. VESICLE small blister · BULLA large blister · PUSTULE contains pus.
4. WHEAL transient and itchy · PURPURA NON-BLANCHING.
5. Record DISTRIBUTION, symmetry, surface and itch.
6. ALWAYS test whether a rash BLANCHES.
7. ECZEMA: itchy, ill-defined, FLEXURES in children, atopic history.
8. PSORIASIS: demarcated plaques, SILVERY scale, EXTENSORS, nails, scalp.
9. Psoriasis may be accompanied by an inflammatory arthritis.
10. SCABIES: intense itch WORSE AT NIGHT, finger WEBS and wrists.
11. Ask whether ANYONE ELSE IN THE HOUSE is itching.
12. Treat ALL household contacts simultaneously.
13. Itch may persist for WEEKS after successful scabies treatment.
14. LEPROSY cardinal signs: patch with DEFINITE SENSORY LOSS.
15. Also THICKENED peripheral nerves and a positive skin smear.
16. TEST SENSATION IN EVERY HYPOPIGMENTED PATCH.
17. Palpate peripheral nerves for thickening.
18. Leprosy disability comes from NERVE DAMAGE, not the organism.
19. Anaesthetic hands and feet are injured unnoticed → ulcers and deformity.
20. Leprosy is CURABLE — regimens from the national programme.
21. FUNGAL: annular, ACTIVE RAISED EDGE, central clearing, itchy.
22. CELLULITIS: spreading hot red tender area — find the portal of entry.
23. PAIN OUT OF PROPORTION in cellulitis = NECROTISING infection.
24. HERPES ZOSTER: painful vesicles in ONE DERMATOME, not crossing the midline.
25. NON-BLANCHING rash with FEVER = meningococcal disease — antibiotics now.
26. SEVERE DRUG REACTION: MUCOSAL involvement, blistering, systemic illness.
27. STOP THE SUSPECT DRUG IMMEDIATELY.
28. Drug rashes appear days to weeks after starting the drug.
29. ERYTHRODERMA: fluid, heat and protein loss plus infection risk — admit.
30. Emollients help both eczema and psoriasis.
💡 Exam angle: three moves cover most of this chapter. Describe the lesion in proper terms and note where it is. Test sensation in any pale patch. And in any rash with fever, check whether it blanches and whether the mucosae are involved — those two questions identify almost every dermatological emergency.
Syllabus points
The vocabulary of skin lesions
Distribution, surface and itch
Why blanching matters
Eczema and the flexures
Psoriasis and the extensors
Scabies and the household
The cardinal signs of leprosy
Testing sensation in a pale patch
Why leprosy causes deformity
Fungal infection and the active edge
Cellulitis and necrotising infection
Herpes zoster and the dermatome
Severe drug reactions and mucosal involvement
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