  This note concerns an extension of the good-$\lambda$ inequality for
fractional integrals, due to B. Muckenhoupt and R. Wheeden. The classical
result is refined in two aspects. Firstly, general nonlinear potentials are
considered; and secondly, the constant in the inequality is proven to decay
exponentially. As a consequence, the exponential integrability of the gradient
of solutions to certain quasilinear elliptic equations is deduced. This in turn
is a consequence of certain Morrey space embeddings which extend classical
results for the Riesz potential.
  In addition, the good-$\lambda$ inequality proved here provides an elementary
proof of the result of Jawerth, Perez and Welland regarding the positive cone
in certain weighted Triebel-Lizorkin spaces.
