Skin conditions and important associations

Acanthosis nigricans -- Insulin resistance (Polycystic Ovarian Syndrome), Gastrointestinal malignancy

Multiple skin tags   -- Insulin resistance , Pregnancy and Crohn disease (perianal)

Porphyria cutanea tarda
Cutaneous leukocytoclastic vasculitis (palpable purpura) secondary to cryoglobulinemia, Lichen planus --
Hepatitis C

Dermatitis herpetiformis  -- Celiac disease

Sudden-onset severe psoriasis 
Recurrent herpes zoster 
Disseminated molluscum contagiosum   -- HIV infection

Severe seborrheic dermatitis  -- HIV infection and Parkinson disease

Explosive onset of multiple itchy seborrheic keratosis -- GI malignancy

Pyoderma gangrenosum -- IBD (inflammatory bowel disease)

Vitiligo -- other autoimmune conditions such as Celiac disease, Pernicious anemia , autoimmune thyroid disease, type 1 diabetes mellitus, primary adrenal insufficiency, hypopituitarism, and alopecia areata.