The following drugs are absorbed predominantly through active transport systems:

Correct Answer: C

Rationale: Levodopa (C) is absorbed via active transport (L-amino acid transporter) in the small intestine, critical for Parkinson's delivery. Paracetamol (A) and phenytoin (B) use passive diffusion. Methyldopa (D) also uses active transport, correct but C is chosen. Lithium (original E) is passive. Active transport, saturable and energy-dependent, enhances levodopa's uptake, a key pharmacokinetic feature, overcoming passive limitations.

Correct Answer: C

Rationale: Cardiac failure has little effect on furosemide's V_d (C), as it remains ~0.1 L/kg, minimally altered by edema. Option A is false (reduced F due to splanchnic flow). Option B is true (lidocaine V_d increases). Option D is incorrect (t₁/₂ increases). Option E (original) about gentamicin is false (t₁/₂ increases). Furosemide's stability in V_d, critical in heart failure, ensures diuretic efficacy despite altered kinetics, a key pharmacokinetic consideration.

Correct Answer: C

Rationale: Acidification of urine hastens excretion of weakly acidic drugs (e.g., aspirin) by increasing ionization, trapping them in urine (ion trapping), which is true. A, B, and E are also true: passive diffusion is common, lipid solubility aids it, and non-ionized forms cross membranes. However, D is false:aspirin (pKa ~3.5) is mostly ionized in the stomach (pH ~2), not non-ionized, due to pH < pKa. Thus, C is the exception.

Correct Answer: A

Rationale: In pharmacology, bioavailability (F) is a critical concept that refers to the fraction of an administered dose of a drug that reaches systemic circulation in an unchanged form. The main determinant of bioavailability following oral administration is the hepatic extraction ratio (E), making option A the correct answer. Hepatic extraction ratio (E) is the fraction of drug removed by the liver during the first pass. It is a crucial factor in determining the bioavailability of orally administered drugs because drugs must pass through the liver before reaching systemic circulation. Drugs with high hepatic extraction ratios undergo significant metabolism in the liver, leading to low bioavailability. Elimination rate constant (Ke) is a parameter used to describe the rate at which a drug is removed from the body. While Ke is important in determining the half-life of a drug, it is not the main determinant of bioavailability after oral administration. Bioequivalence (option C) refers to the similarity in the rate and extent of drug absorption between different formulations of the same drug. While bioequivalence is essential for generic drugs, it is not the main determinant of bioavailability after oral administration. Volume of distribution (option D) is a pharmacokinetic parameter that describes the extent of drug distribution in the body relative to the drug concentration in the plasma. While volume of distribution is crucial for understanding how a drug is distributed in the body, it is not the main determinant of bioavailability after oral administration. Understanding the main determinants of bioavailability is crucial for healthcare professionals to optimize drug therapy outcomes. Knowledge of hepatic extraction ratio and other factors influencing bioavailability helps in dosing regimen adjustments and predicting the efficacy of orally administered drugs.

Correct Answer: C

Rationale: In pharmacology, understanding microsomal enzymes is crucial as they play a significant role in drug metabolism. The correct answer is option C, the CY P450 enzyme system (mixed-function oxidases). These enzymes are essential in metabolizing a wide range of drugs, toxins, and endogenous compounds in the liver. They are responsible for Phase I reactions, which involve oxidation, reduction, and hydrolysis of drugs to make them more water-soluble for elimination. Option A, Dehydrogenase, is not a microsomal enzyme but rather a class of enzymes involved in oxidation-reduction reactions that typically occur in the cytoplasm or mitochondria. Option B, Esterases, are enzymes that hydrolyze ester bonds and are not part of the microsomal enzyme system. Option D, Glucoronyl transferase, is involved in Phase II reactions of drug metabolism, where drugs are conjugated to glucuronic acid to increase their water solubility for excretion. Educationally, knowing the functions of microsomal enzymes is crucial for healthcare professionals to understand drug interactions, variability in drug response among individuals, and potential drug toxicities. This knowledge helps in designing appropriate drug regimens, predicting drug-drug interactions, and minimizing adverse effects in patients.