What is the reason of complicated penetration of some drugs through brain-blood barrier?

Correct Answer: C

Rationale: The correct answer is C: Absence of pores in the brain capillary endothelium. The brain-blood barrier (BBB) is a selective barrier that prevents many substances from freely entering the brain. The brain capillary endothelium does not have pores like other capillaries in the body, making it impermeable to large molecules and pathogens. This selective permeability is crucial for maintaining the brain's delicate environment. Choices A, B, and D are incorrect because high lipid solubility of a drug, meningitis, and high endocytosis degree in a brain capillary do not directly relate to the complicated penetration of drugs through the BBB.

Correct Answer: B

Rationale: The correct answer is B: Bupivacaine. Bupivacaine is more cardiotoxic compared to the other local anesthetics listed. This is due to its higher potency in blocking cardiac sodium channels, leading to a higher risk of cardiac toxicity, including arrhythmias and cardiac arrest. Procaine, lidocaine, and mepivacaine are less cardiotoxic as they have lower affinity for cardiac sodium channels and are less likely to cause significant cardiac adverse effects. It is important to be cautious when using bupivacaine to minimize the risk of cardiotoxicity.

Correct Answer: B

Rationale: The correct answer is B: Competitive muscarinic blockade. Atropine is an anticholinergic drug that works by competitively blocking muscarinic receptors, leading to inhibition of parasympathetic nervous system activity. This results in effects such as increased heart rate and dilated pupils. A: Competitive ganglion blockade is incorrect as atropine does not act on ganglionic receptors. C: Competitive neuromuscular blockade is incorrect as atropine does not affect neuromuscular junctions. D: Noncompetitive neuromuscular blockade is incorrect as atropine does not act in a noncompetitive manner on neuromuscular receptors.

Correct Answer: A

Rationale: The correct answer is A: Myasthenia gravis. In myasthenia gravis, there is a decrease in the number of available acetylcholine receptors at the neuromuscular junction, leading to muscle weakness. Nondepolarizing muscle relaxants work by blocking acetylcholine receptors, so in myasthenia gravis, there are fewer receptors available for the drug to bind to, resulting in an augmented neuromuscular blockade. Burns (B), asthma (C), and Parkinsonism (D) do not directly affect the number of acetylcholine receptors at the neuromuscular junction, so they would not enhance the blockade produced by nondepolarizing muscle relaxants.

Correct Answer: A

Rationale: Step-by-step rationale: 1. Epinephrine is a sympathomimetic agent that constricts blood vessels, reduces bleeding, and prolongs the effect of local anesthetics. 2. By vasoconstriction, epinephrine delays systemic absorption of the anesthetic, leading to a longer duration of action. 3. Xylometazoline is a nasal decongestant, not used for nerve blocks. 4. Isoproterenol and dobutamine are primarily used for cardiac conditions, not for local anesthesia. Summary: Epinephrine is the correct choice due to its vasoconstrictive properties prolonging the effect of local anesthetics. Xylometazoline, Isoproterenol, and Dobutamine are incorrect as they do not serve the same purpose in nerve blocks.