Which of the following drugs does not pass the blood-brain barrier:

Correct Answer: D

Rationale: The correct answer is D: Guanethidine. Guanethidine is a quaternary amine, which prevents it from passing the blood-brain barrier due to its charge. The blood-brain barrier is selective and only allows certain molecules to pass through. In contrast, choices A, B, and C (Ephedrine, Propranolol, Clonidine) are all capable of crossing the blood-brain barrier to some extent due to their chemical properties. Ephedrine is a weak base, Propranolol is lipophilic, and Clonidine is relatively small in size, allowing them to penetrate the blood-brain barrier.

Correct Answer: D

Rationale: The correct answer is D: Ergot alkaloids. Ergot alkaloids, such as ergotamine and dihydroergotamine, are known to be effective in treating migraines by constricting blood vessels in the brain. This helps to reduce the intensity of migraine headaches. Vinca alkaloids (A) are used in cancer treatment, digitalis glycosides (B) in heart conditions, and stramonium alkaloids (C) in asthma and spasms, but they are not commonly associated with migraine treatment.

Correct Answer: D

Rationale: The correct answer is D: Propranolol. Propranolol is a non-selective beta-blocker that can trigger severe bronchospasm in patients with asthma by blocking beta-2 receptors in the lungs, leading to bronchoconstriction. Atropine (A) is an anticholinergic drug that dilates the airways. Isoproterenol (B) is a beta-agonist that relaxes the airway smooth muscles. Phenylephrine (C) is a sympathomimetic drug that constricts blood vessels, not airways.

Correct Answer: C

Rationale: Step 1: Hyoscine is an antimuscarinic drug used for motion sickness. Step 2: Atropine is a similar prototype drug as it also belongs to the antimuscarinic class. Step 3: Atropine, like hyoscine, can cause photophobia and blurred vision. Step 4: Monoamine oxidase inhibitors (A) are not related to antimuscarinic drugs. Step 5: Propranolol (B) is a beta-blocker and not an antimuscarinic drug. Step 6: Bethanechol (D) is a muscarinic agonist, opposite in action to antimuscarinic drugs like hyoscine and atropine.

Correct Answer: C

Rationale: The correct answer is C because diagram X shows an increase in oxygen concentration over time, which indicates photosynthesis occurring. In photosynthesis, plants take in carbon dioxide and release oxygen. Therefore, the conditions of oxygen at 25°C and carbon dioxide at 25°C would support this process. Choices A, B, and D have incorrect combinations of temperature and gases that would not result in the observed increase in oxygen concentration in diagram X.