Correct Answer Node:

Academic Rationale:

Passive diffusion depends on the ability of a drug to cross lipid bilayers. According to Fick's law, diffusion rate is directly proportional to lipid solubility, concentration gradient, and surface area. Lipid-soluble drugs cross membranes more easily than water-soluble drugs. Plasma protein binding and renal clearance do not directly affect membrane diffusion.

Correct Answer Node:

Academic Rationale:

Weak bases remain largely unionized in alkaline pH (as per Henderson-Hasselbalch equation). The intestine has an alkaline environment (pH ~6–8), favoring unionized form, which is more lipid-soluble and easily absorbed. Hence both assertion and reason are correct and causally related.

Correct Answer Node:

Academic Rationale:

Bioavailability refers to the fraction of administered drug reaching systemic circulation (b). Volume of distribution is the apparent volume in which drug distributes (c). Clearance is defined as the volume of plasma cleared of drug per unit time (a). Hence 1-b, 2-c, 3-a.

Correct Answer Node:

Academic Rationale:

First-pass metabolism occurs in liver after oral absorption via portal circulation. Sublingual route directly enters systemic circulation via venous drainage, bypassing hepatic metabolism. Rectal route partially bypasses, but upper rectum still drains into portal circulation.

Correct Answer Node:

Academic Rationale:

Only unbound (free) drug can cross membranes and interact with receptors. Protein-bound drug acts as a reservoir and cannot diffuse across membranes. Binding prolongs duration by slowly releasing free drug, maintaining plasma concentration.