If $\int \left( \frac{1 - 5\cos^2 x}{\sin^5 x \cos^2 x} \right) dx = f(x) + C$, where $C$ is the constant of integration, then $f\left(\frac{\pi}{6}\right) - f\left(\frac{\pi}{4}\right)$ is equal to:
\( \dfrac{4}{\sqrt{3}} (8 - \sqrt{6}) \)
\( \dfrac{1}{\sqrt{3}} (26 + \sqrt{3}) \)
\( \dfrac{2}{\sqrt{3}} (4 + \sqrt{6}) \)
\( \dfrac{1}{\sqrt{3}} (26 - \sqrt{3}) \)