  We show that the exclusive decay $B\rightarrow K^{\ast}\gamma$ can be related
to the semileptonic decay $B\rightarrow\rho e\bar{\nu}$ using heavy-quark
symmetry and $SU(3)$ flavor symmetry. A direct measurement of the
$q^{2}$-spectrum for the semileptonic decay can provide relevant information
for the exclusive rare decay.
