What lesion differentiates scabies from other rashes?

A curved or linear burrow about 1 to 10 mm long (caused by the movement of the mite in the stratum corneum) that often ends in a tiny papule or pustule.



Source

Marco, C. and Soleimani, M.  "Cutaneous Infestations"  Critical Decisions in Emergency Medicine.  Jan 2013.

Image Source: http://www.bpac.org.nz/magazine/2009/february/scabies.asp