Genodermatoses — CUTICON Prelim Quiz Prep
1. Topic-Wise Study Notes
A. Ichthyoses (very high-yield, easy image spots)
| Disease | Inheritance/Gene | Key Feature |
|---|
| Ichthyosis vulgaris | AD, FLG (filaggrin) | Commonest ichthyosis; fine scales on extensors, spares flexures; associated with atopic dermatitis, keratosis pilaris |
| X-linked recessive ichthyosis | XLR, STS (steroid sulfatase) | Large brown scales; corneal opacities; mothers may have failure of labor (placental sulfatase deficiency) |
| Lamellar ichthyosis / CIE (ARCI spectrum) | AR, TGM1 most common | Collodion baby at birth; large plate-like scales |
| Harlequin ichthyosis | AR, ABCA12 | Most severe; thick armor-like plates, ectropion/eclabium, often neonatal death without treatment |
| Epidermolytic ichthyosis | AD, KRT1/KRT10 | Blistering at birth, later thick verrucous scale; histology shows epidermolytic hyperkeratosis |
| Netherton syndrome | AR, SPINK5 | Triad: ichthyosis linearis circumflexa, trichorrhexis invaginata ("bamboo hair"), atopy/immune defects |
| Sjögren-Larsson syndrome | AR, ALDH3A2 | Ichthyosis + spasticity + intellectual disability |
(Source: Dermatology 2-Vol Set 5e, Ch. 56-57; Fitzpatrick's Dermatology 9e)
B. Epidermolysis Bullosa (mechanobullous group)
Classified by cleavage plane — 4 major types:
- EB simplex — intraepidermal, KRT5/KRT14 — mildest, rarely scars
- Junctional EB — lamina lucida, laminin-332/collagen XVII — severe, pyloric atresia in some
- Dystrophic EB — sublamina densa, COL7A1 (type VII collagen) — scarring, milia, nail loss; recessive form most severe (mitten deformity, high SCC risk)
- Kindler syndrome — mixed cleavage planes, FERMT1 (kindlin-1) — poikiloderma + photosensitivity
Extracutaneous risks to remember for quiz: GERD, anemia, esophageal strictures, squamous cell carcinoma (RDEB — leading cause of death in adults with RDEB).
C. Neurocutaneous Syndromes (phakomatoses)
Neurofibromatosis type 1 (chromosome 17, NF1 gene → neurofibromin, a RAS-GAP tumor suppressor)
Diagnosis needs ≥2 of 7:
- ≥6 café-au-lait macules (>5mm prepubertal / >15mm postpubertal)
- ≥2 neurofibromas of any type, or 1 plexiform neurofibroma
- Axillary/inguinal freckling (Crowe sign)
- Optic pathway glioma
- ≥2 Lisch nodules (iris hamartomas)
- Distinctive osseous lesion (sphenoid wing dysplasia, tibial pseudarthrosis)
- First-degree relative with NF1
Neurofibromatosis type 2 (chromosome 22, NF2 gene → merlin) — bilateral vestibular schwannomas is the hallmark.
Tuberous sclerosis complex (TSC1-hamartin or TSC2-tuberin, mTOR pathway)
Cutaneous major features: hypomelanotic macules (≥3), facial angiofibromas (≥3) or fibrous cephalic plaque, ungual fibromas (≥2), shagreen patch.
Other majors: subependymal nodules, SEGA, cardiac rhabdomyoma, retinal hamartomas, lymphangioleiomyomatosis, angiomyolipomas (≥2).
Minor: "confetti" skin lesions, dental enamel pits (>3), intraoral fibromas.
Definite diagnosis = 2 major, or 1 major + 2 minor (2012 revised criteria).
D. DNA Repair Defect
Xeroderma pigmentosum — AR, defective nucleotide excision repair (multiple XP complementation genes). Extreme photosensitivity from infancy, early freckling, high risk of BCC/SCC/melanoma at a young age, some variants with neurological degeneration (De Sanctis-Cacchione syndrome).
E. Other quiz-favorites
- Pachyonychia congenita — AD, keratin 6A/6B/6C/16/17 — painful palmoplantar keratoderma + nail dystrophy
- Palmoplantar keratodermas — divided into diffuse, focal, punctate; some syndromic (Papillon-Lefèvre with periodontitis, Vohwinkel with pseudoainhum)
- Incontinentia pigmenti — X-linked dominant, NEMO/IKBKG, lethal in males in utero; 4 stages (vesicular → verrucous → hyperpigmented whorls → hypopigmented atrophic streaks)
2. Practice Q&A
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Q: Bamboo hair on microscopy with ichthyosis and atopy points to which syndrome, and which gene?
A: Netherton syndrome; SPINK5 mutation (trichorrhexis invaginata).
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Q: A neonate born with thick, armor-plate-like skin, ectropion, and eclabium. Diagnosis and gene?
A: Harlequin ichthyosis; ABCA12 mutation.
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Q: Number of café-au-lait macules needed for NF1 diagnosis in a prepubertal child, and minimum size?
A: ≥6 macules, each >5 mm in prepubertal patients (>15 mm postpubertal).
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Q: Which EB subtype carries the highest lifetime risk of cutaneous squamous cell carcinoma?
A: Recessive dystrophic EB (RDEB).
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Q: Chromosome and protein defective in NF2?
A: Chromosome 22; merlin (schwannomin).
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Q: A collodion baby that later develops large, plate-like scales without erythroderma — most likely diagnosis?
A: Lamellar ichthyosis (ARCI spectrum, commonly TGM1).
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Q: TSC diagnostic criteria — name any 2 major cutaneous features.
A: Facial angiofibromas (≥3) / fibrous cephalic plaque, hypomelanotic macules (≥3), shagreen patch, ungual fibromas (≥2) — any two.
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Q: DNA repair pathway defective in xeroderma pigmentosum?
A: Nucleotide excision repair (NER).
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Q: X-linked recessive ichthyosis is due to deficiency of which enzyme, and what maternal clue may be found at delivery?
A: Steroid sulfatase deficiency; low estriol / failure of labor progression due to placental sulfatase deficiency.
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Q: Kindler syndrome gene and its distinguishing clinical feature versus other EB types?
A: FERMT1 (kindlin-1); mixed-level blistering plus progressive poikiloderma and photosensitivity (not typical of other EB types).
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Q: Incontinentia pigmenti — inheritance pattern and why is it rarely seen in males?
A: X-linked dominant, NEMO/IKBKG gene; typically lethal in hemizygous males in utero.
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Q: Which keratin genes are mutated in epidermolytic ichthyosis, and what is the classic histology?
A: KRT1/KRT10; epidermolytic hyperkeratosis (vacuolization of upper epidermis with clumped keratohyalin).
3. Quiz Strategy Tips (genodermatoses-specific)
- Image rounds decide these questions — genodermatoses are almost always tested via clinical photos (café-au-lait macules, shagreen patch, collodion baby, harlequin baby, Bruton's sign/Crowe sign). Practice spot recognition, not just gene names.
- Anchor each disease to ONE defining gene/protein — quiz writers love "gene-disease" matching (e.g., FLG-ichthyosis vulgaris, COL7A1-DEB, NF1-neurofibromin, TSC1/2-mTOR). Make a one-line flash association for each.
- Know the "rule of criteria" for NF1 and TSC cold — these numeric diagnostic criteria are extremely popular in prelim MCQs.
- Watch for buzzwords: "bamboo hair" = Netherton, "collodion baby" = ARCI, "Crowe sign" = NF1, "shagreen patch" = TSC, "mitten deformity" = RDEB, "shagreen" vs "peau d'orange" (don't confuse with scleroderma).
- Cross-link to other specialties — genodermatoses questions often blend with pediatrics/neurology (seizures in TSC, acoustic neuroma in NF2), so expect integrative questions, not pure derm recall.
- Don't neglect inheritance pattern questions — a quick table of AD vs AR vs X-linked for each disease is high-yield and easy to score.
4. Recent Updates (2023-2025)
- Beremagene geperpavec (Vyjuvek) — FDA-approved (2023) topical HSV-1 vector-based gene therapy delivering functional COL7A1 for dystrophic EB wounds; first in-vivo redosable gene therapy for a genodermatosis — a strong "recent advances" quiz question.
- Birch triterpenes topical gel (Filsuvez) — FDA-approved (Dec 2023) for EB wound care, promotes wound closure.
- Broader move toward CRISPR-based gene editing for genodermatoses (EB, ichthyoses) is an active review area — challenges include delivery, mosaicism correction, and long-term durability (Piñón Hofbauer et al. 2024; Brooks et al. 2023).
- Increasing emphasis on molecular/genomic classification replacing older purely clinical classification systems (exome sequencing as first-line diagnostic tool) — Dermatology 5e now organizes genodermatoses by pathomechanism (keratin defects, RASopathies, etc.) rather than morphology alone.
- No new randomized trial evidence changes textbook management fundamentally; the shift is diagnostic (genomic) and in targeted/gene therapies rather than new systemic drugs.
Want me to move to the next topic (leprosy, bullous disorders, or psoriasis/biologics), or drill deeper into any genodermatoses subtopic (e.g., more MCQs, or a dedicated set of image-based spotters)?