5. For the central conic section $\frac{x^{2}}{m}+\frac{y^{2}}{n}=1(m>|n|>0)$, there is a point $P$ on it such that the lines connecting $P$ to the two foci $F_{1}$ and $F_{2}$ are perpendicular to each other. Then the area of $\mathrm{Rt} \triangle P F_{1} F_{2}$ is $(\quad)$.
(A) $|n|$
(B) $m$
(C) $n^{2}$
(D) Uncertain
5.A.
$n>0$, the curve is an ellipse. Then
$$
\begin{array}{l}
\left|P F_{1}\right|+\left|P F_{2}\right|=2 \sqrt{m}, \\
P F_{1}^{2}+P F_{2}^{2}=4(m-n) .
\end{array}
$$
(1)$^{2}$ - (2) gives
$$
\left|P F_{1}\right|\left|P F_{2}\right|=2 n, S_{\triangle P F_{1} F_{2}}=|n| \text {. }
$$

Similarly, when $n<0$, $S_{\triangle P F_{1} F_{2}}=|n|$.