Learning Objectives
Differentiate between systemic markers in the skin, such as Acanthosis Nigricans and Erythema Nodosum. Master the “6 P’s” and histological “sawtooth” pattern of Lichen Planus, and identify the classic “Christmas tree” distribution of Pityriasis Rosea.
1. Systemic and Metabolic Markers
These disorders are often the first visible signs of internal metabolic dysfunction, malignancy, or systemic inflammation.
| Condition | Pathophysiology / Distribution | Systemic Associations |
|---|---|---|
| Acanthosis Nigricans | Epidermal hyperplasia; hyperpigmented, velvety thickening in axilla/neck. | Insulin resistance (Diabetes, Obesity, PCOS) or Visceral malignancy (Gastric adenocarcinoma). |
| Erythema Nodosum | Panniculitis (inflammation of subcutaneous fat); painful nodules on the anterior shins. | Sarcoidosis, IBD, Coccidioidomycosis, TB, and Strep infections. |
2. Papulosquamous & Desquamation Disorders
These conditions involve distinct patterns of scaling and inflammation, often with characteristic clinical distributions or genetic origins.
| Condition | Clinical / Histological Hallmark | Key Diagnostic Clue |
|---|---|---|
| Lichen Planus | 6 P’s; Sawtooth infiltrate of lymphocytes; Wickham striae (mucosal). | Strong association with Hepatitis C. |
| Pityriasis Rosea | “Herald patch” followed by “Christmas tree” distribution on trunk. | Self-resolving in 6–8 weeks; collarette scale. |
| Ichthyosis Vulgaris | Defective desquamation due to Filaggrin mutation; diffuse scaling. | Associated with Atopic dermatitis; presents in infancy. |
3. UV and Radiation-Induced Injury
Skin damage from high-energy radiation can be acute (apoptosis-driven) or late (fibrosis-driven).
| Condition | Mechanism | Presentation Timeline |
|---|---|---|
| Sunburn | UVB causes DNA damage and keratinocyte apoptosis. | Acute inflammatory reaction. |
| Radiation Dermatitis | Acute: Basal keratinocyte apoptosis. Late: Dermal fibrosis and telangiectasias. |
Acute: ≤ 90 days. Late: Months to years post-radiotherapy. |
Activity
High-Yield Mnemonics & Tips:
- Lichen Planus 6 P’s: Pruritic, Purple, Polygonal, Planar, Papules, Plaques.
- Sunburn vs. Aging: UVB for Burn; UVA for Aging and tAnning.
- Acanthosis Nigricans: If it appears suddenly in an older patient who isn’t obese, look for Gastric Cancer.
- Lichen Planus Histology: The Sawtooth pattern at the dermal-epidermal junction is pathognomonic.