Vorticity and isotropic singularities

We prove that if a solution of the Einstein field equations with perfect fluid source and law equation of state p = (–1)] admits an Isotropic singularity, then necessarily the fluid flow is irrotational. This shows the essential equivalence of the seemingly distinct concepts of quasi-isotropic singularities and Friedrnann-like singularities of Lifshitz and Khalatnikov and Eardley, Liang and Sachs, respectively.