Correct Answer � A Explanation

Benign Skin Tumours: Dermatofibroma — Fitzpatrick’s Sign

Fitzpatrick’s sign (dimpling on lateral compression) is pathognomonic of dermatofibroma; the peripheral pigment network on dermoscopy is also characteristic.

Core Concept

• Dermatofibroma (DF) (fibrous histiocytoma, sclerosing haemangioma):
• Benign fibrohistiocytic tumour of the dermis; most common on lower limbs; also arms, trunk
• Common in middle-aged women
• Gross: firm, slightly raised, flesh-coloured to light brown papule/nodule, often with central dimple
• Histology: spindle-shaped histiocytes (fibroblast-like) + collagen bundles; overlying epidermal hyperplasia with basal hyperpigmentation (explains peripheral pigment network on dermoscopy)
• Fitzpatrick’s dimple sign: lateral compression of skin around DF → skin dimples because the DF is tethered to the overlying epidermis; UNIQUE to DF
• Dermoscopy: peripheral delicate pigment network (reticular pattern), central white patch (scar-like)
• Possible trigger: minor trauma, insect bite
• Malignant potential: extremely rare; dermatofibrosarcoma protuberans (DFSP) is the malignant counterpart
• Treatment: observation; excision if symptomatic or diagnostic doubt
• Lipoma: soft, lobulated, subcutaneous, not tethered to skin → no dimple sign; no peripheral pigment network; moves freely
• Epidermal inclusion cyst: smooth, dome-shaped, central punctum; not tethered in the Fitzpatrick sense; may discharge cheesy material
• Neurofibroma: soft, button-hole sign (invaginates into dermis on pressure); NF1-associated; bag-of-worms texture if plexiform

Why A (dermatofibroma) is correct

• Fitzpatrick’s dimple sign + peripheral pigment network on dermoscopy + firm nodule = dermatofibroma
• No other benign skin tumour has a positive Fitzpatrick’s sign

Why the Other Options Are Wrong

B. Lipoma — lipoma is soft and fluctuant, not firm; it is NOT tethered to the skin; Fitzpatrick’s sign is NEGATIVE; no pigment network on dermoscopy. Discriminator: Soft, fluctuant, NOT tethered = lipoma; firm, tethered, Fitzpatrick+ = dermatofibroma.

C. Epidermal inclusion cyst — epidermal cyst has a central punctum; is soft-firm and fluctuant; may rupture and become inflamed; no Fitzpatrick’s sign; no peripheral pigment network. Discriminator: Central punctum + cystic = inclusion cyst; Fitzpatrick+ = dermatofibroma.

D. Neurofibroma — neurofibroma: button-hole sign (easy invagination into dermis); plexiform type has bag-of-worms texture; NF1 café-au-lait spots; dermoscopy: pink structureless areas; no Fitzpatrick’s dimpling. Discriminator: Button-hole invagination = neurofibroma; dimpling on lateral compression = dermatofibroma.

High-yield Recap

• Dermatofibroma: Fitzpatrick’s dimple sign + peripheral pigment network on dermoscopy
• Common site: lower limbs in women; often post-trauma
• Histology: spindle cells + collagen; overlying epidermal hyperplasia
• DFSP (malignant counterpart): storiform spindle cells, CD34+, t(17;22) COL1A1-PDGFB
• Button-hole sign: neurofibroma (NF1 association)
• Lipoma: soft, lobulated, no sign; no pigment network