  We report the observation of a nonlinear elastoresistivity response for the
prototypical underdoped iron pnictide Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$.
Our measurements reveal a large quadratic term in the isotropic ($A_{1g}$)
electronic response that was produced by a purely shear ($B_{2g}$) strain. The
divergence of this quantity upon cooling towards the structural phase
transition reflects the temperature dependence of the nematic susceptibility.
This observation shows that nematic fluctuations play a significant role in
determining even the isotropic properties of this family of compounds.
