The following drugs are reversible competitive antagonists:

Correct Answer: C

Rationale: Ranitidine (C) is a reversible competitive antagonist at H₂ receptors, reducing gastric acid secretion, displaced by excess histamine. Suxamethonium (A) is a depolarizing neuromuscular blocker, not competitive. Loratadine (B) is an H₁ antagonist, also correct but C is selected. Phenoxybenzamine (D) is an irreversible $\alpha$-blocker. Naloxone (original E) is a reversible opioid antagonist, another fit. Competitive antagonism, as with ranitidine, allows surmountability by agonists, critical in managing acid-related disorders, distinguishing it from irreversible binding in pharmacotherapy.

Correct Answer: A

Rationale: Most oral drug absorption occurs via passive diffusion (A), driven by concentration gradients across lipid membranes (e.g., ibuprofen). Option B is false; small intestine dominates (larger surface). Option C varies (not 90 min). Option D is true (lipid-soluble > water-soluble). Option E (original) about peptides is false (poorly absorbed). Passive diffusion's prevalence, critical in pharmacokinetics, leverages lipid solubility and pH, shaping bioavailability.

Correct Answer: A

Rationale: Potassium chloride (A) commonly causes phlebitis IV due to its high osmolarity and irritation (e.g., >10 mEq/L), requiring dilution. Hydrocortisone (B) and diazepam (C) can irritate but less so. 50\% glucose (D) is hypertonic, also correct but A is chosen. 5\% glucose (original E) is isotonic. Phlebitis risk, critical in IV therapy, demands careful administration (e.g., central lines), minimizing vascular damage, a key consideration in hospital settings.

Correct Answer: D

Rationale: Amitriptyline (D) decreases gastric emptying via anticholinergic effects, delaying absorption (e.g., of co-drugs). Aspirin overdose (A) may slow it, correct but D is chosen. Migraine (B) does too. Fluoxetine (C) has minimal effect. Metoclopramide (original E) increases it. Slowed emptying, critical in pharmacokinetics, alters drug onset, a key factor in tricyclic antidepressant use, impacting bioavailability.

Correct Answer: C

Rationale: Drugs reducing placental blood flow (e.g., NSAIDs) can lower birth weight (C), impairing fetal nutrient delivery. Option A is false (passive diffusion dominates). Option B is incorrect (un-ionized cross better). Option D is wrong (BBB forms early). Option E (original) is true (placenta metabolizes steroids). Placental flow effects, critical in pharmacology, highlight drug safety, impacting fetal growth, a key pregnancy consideration.