These are differentials that you will use over and over again for common presentations. Drill them until they are automatic, and go through the whole list in your head when you are presented with the appropriate patient. Otherwise, it is easy to get lazy and then miss something that might have been otherwise obvious.
Some of the differentials will include an "expanded differential" - this lists diseases that may be less useful to you either because they are "rare" or because they are "distinct" (meaning they have such a distinct presentation that they are usually easy to diagnose don't really have a differential diagnosis).
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This is a common differential for a hospital consult (or for any patient who has spent time with their back occluded, usually from being bed-bound).
CLASSIC DIFFERENTIAL DIAGNOSES:
| • | Sweaty back: candida vs. folliculitis vs. miliaria vs. Grover’s disease vs. “doubt drug” |
| • | superficial pustules: candida vs. impetigo vs. Sneddon-Wilkinson vs. pustular psoriasis/ AGEP |
| • | tender red hands in a cancer patient: GVHD vs. acral erythema (of chemotherapy) vs. neutrophilic eccrine hidradenitis |
| • | reticulate erythema: erythema infectiosum vs. Still’s disease vs. erythema marginatum (vs. generalized LE) |
| • | annular erythema: EAC vs. erythema migrans vs. erythema gyratum repens |
| • | suppurative and verrucous: pyoderma gangrenosum vs. halogenoderma vs. deep fungal |
| • | verrucous purple on legs: hyperkeratotic LP/KLC vs. KS/pseudo-KS (acroangiodermatitis) vs. Gougerot-Blum disease |
| • | flaky paint: (necrolytic erythemas) acrodermatitis enteropathica (zinc) vs. necrolytic migratory erythema (glucagonoma) vs. pellagra (niacin) vs. fatty acid deficiency vs. ichthyosislinearis circumflexa vs. p. foliaceous |
| • | friable nodule on elderly scalp: angiosarcoma vs. AFX vs. SCC vs. Merkel Cell tumor vs. mets |
SHORT, SIMPLE, AUTOMATIC: if you think of one, you should automatically think of the other(s)
| • | spitz nevus vs. JXG vs. urticaria pigmentosa |
| • | swimmers itch vs. sea bathers eruption |
| • | wart vs. clavus |
| • | neurofibroma vs. intradermal nevus vs. skin tag |
| • | PR vs. RPR secondary syphilis |
| • | acanthosis nigricans vs. Gougerot Carteaud vs. Dowling Degos disease (vs. TV vs. ashy dermatosis) |
| • | ILVEN vs. lichen striatus |
| • | Marfan's syndrome vs. homocystinuria |
| • | nevus lipomatosis superficialis vs. collagenoma vs. plexiform neurofibroma |
| • | necrolytic migratory erythema vs. acrodermatitis enteropathica |
| • | PLEVA vs. PLC vs. lymphomatoid papulosis |
| • | erythema marginatum vs. Still’s disease* |
| • | tumid LE vs. REM* |
| • | lupus profundus vs. subcutaneous T-cell lymphoma |
| • | AN vs. Gougerot-Carteaud |
| • | PR vs. EAC (histology and ~ morphology) |
| • | EAC (deep gyrate erythema) “vs.” or “is” erythema gyratum repens (identical histologically) |
| • | pyogenic granuloma vs. bacillary angiomatosis |
| • | Sweet's syndrome vs. atypical pyoderma gangrenosum |
| • | BPP vs. cryoglobulinemia (especially if Schamberg’s in a female) - classically BPP with small ulcers vs.Waldenstrom’s hypergammaglobulinemia Cayenne pepper |
* see also Cousins