Twin Sisters with an Odd, Progressive Rash

A first-time mother comes in with her twin girls for a well-child visit. The perinatal and postpartum history appears unremarkable until the mother mentions that both girls had a rash on trunk and extremities immediately after birth that lasted a few weeks. The rash progressed in the same distinct manner on both girls.Dr Jon Schneider narrates the history in the slide show above which includes images of the twins as they presented at their appointment. What would you include in your differential? What's your diagnosis?       ReferencesBerlin AL, Paller AS, Chan LS. Incontinentia pigmenti: a review and update on the molecular basis of pathophysiology. J Am Acad Dermatol. 2002;47:169-87; quiz 188-90.Minic S, Trpinac D, Obradovic M. Incontinentia pigmenti diagnostic criteria update.  Clinical Genetics: 2014;85:536-542.National Center for Advancing Translational Science, Genetic and Rare Diseases Information Center (GARD). Incontinentia pigmenti. Accessed on June 16, 2016 and available at https://rarediseases.info.nih.gov/gard/6778/incontinentia-pigmenti/resources/1Scheuerle AE, Ursini MV. Incontinentia pigmenti. In: Pagon RA, Adam MP, Ardinger HH, et al., eds. Gene Reviews [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2016. Last Update: February 12, 2015.Swinney CC, Han DP, Karth PA. Incontinentia pigmenti: A comprehensive review and update. Ophthalmic Surg Lasers Imaging Retina. 2015;46:650-657. Posted June 27, 2015. DOI: 10.3928/23258160-20150610-09