• Determinants of Glomerular Filtration: In renal elimination, glomerular filtration of a drug is fundamentally governed by its plasma protein binding.
• Free Drug Fraction: Only the unbound (free) drug fraction in plasma is small enough to pass through the fenestrated glomerular capillary endothelial pores and basement membrane podocyte slit diaphragms ($cutoff \sim 60\text{ kDa}$). Drug molecules bound to large plasma proteins (albumin, $\alpha_1$-acid glycoprotein) cannot be filtered.

Correct option: (2) Plasma protein binding