The following drugs obey non-linear (dose-dependent) elimination pharmacokinetics:

Correct Answer: C

Rationale: Phenytoin exhibits non-linear kinetics because its elimination pathways become saturated at therapeutic doses, leading to a disproportionate rise in plasma levels with dose increases.

Correct Answer: D

Rationale: Sildenafil (Viagra), a phosphodiesterase-5 inhibitor, enhances penile blood flow by increasing cyclic GMP, directly treating erectile dysfunction (ED) and is the standard first-line therapy. Leuprolide, a GnRH agonist, suppresses testosterone for prostate cancer, potentially worsening ED. Finasteride, for benign prostatic hyperplasia (BPH), reduces prostate size but may cause ED as a side effect, not treat it. Tamsulosin, an alpha-blocker for BPH, improves urinary flow but isn't indicated for ED and can lower blood pressure. Sildenafil's targeted action on vascular mechanisms in ED distinguishes it, offering rapid efficacy and a well-established safety profile for this condition.

Correct Answer: D

Rationale: Acetylcysteine is the antidote for acetaminophen (paracetamol) overdose. Acetaminophen overdose can lead to hepatic necrosis, and acetylcysteine is effective in preventing or reversing this hepatotoxicity. It works by replenishing depleted hepatic stores of glutathione, which is essential for detoxifying the reactive metabolite of acetaminophen. This helps to protect the liver cells from damage caused by the toxic metabolite of acetaminophen. Naloxone, vitamin K, warfarin, and diphenhydramine are not effective in reversing the hepatic necrosis caused by paracetamol overdose.

Correct Answer: D

Rationale: The loading dose of a drug required to reach an initial target blood concentration is based on the volume of distribution (Vd) of the drug. The volume of distribution represents the theoretical volume that would be necessary to contain the total amount of the administered drug at the same concentration as in the blood plasma. The loading dose is calculated by multiplying the target blood concentration by the volume of distribution of the drug. This initial dose helps to rapidly achieve the desired drug concentration in the body, especially when a drug has a long half-life or when steady-state levels need to be reached more quickly. Understanding the volume of distribution of a drug is crucial in dosage calculation to ensure therapeutic efficacy and avoid potential toxicity.

Correct Answer: C

Rationale: Bisphosphonates (e.g., alendronate) halt bone loss-bone density scans measure improvement, per efficacy standard. Calcium levels don't reflect bone strength. Fractures or pain absence helps but isn't direct-density shows progress, per treatment.