Your patient is diagnosed with myasthenia gravis, and she is given a prescription for pyridostigmine. Which of the following best summarizes the drug’s mechanism of action?

Correct Answer: D

Rationale: The correct answer is D: "Inhibits breakdown of acetylcholine, thereby improving activation of skeletal muscle by acetylcholine." Rationale: 1. Pyridostigmine is an acetylcholinesterase inhibitor, which means it blocks the breakdown of acetylcholine. 2. In myasthenia gravis, there is a decrease in acetylcholine receptor sites, leading to muscle weakness. 3. By inhibiting the breakdown of acetylcholine, pyridostigmine increases the availability of acetylcholine at the neuromuscular junction, improving muscle activation. 4. This mechanism helps alleviate muscle weakness in myasthenia gravis patients. Summary: A: Incorrect. Pyridostigmine does not block muscarinic receptors or change the preference of acetylcholine stimulation. B: Incorrect. Pyridostigmine does not inhibit reuptake of acetylcholine; it acts by inhibiting its breakdown

Correct Answer: C

Rationale: The correct answer is C: 160 m. To calculate distance traveled, we use the formula: distance = speed x time. In this case, the speed of the car is 8 m/s (given in the graph) and the time is 20 s. Therefore, distance = 8 m/s x 20 s = 160 m. Choice A is incorrect as it is too low. Choice B is incorrect as it does not account for the full 20 s. Choice D is incorrect as it is too high based on the given speed and time.

Correct Answer: B

Rationale: The correct answer is B because steroid hormones bind to DNA and form a gene-hormone complex, which then regulates gene expression. This directly influences cellular activities at the genetic level. Choices A, C, and D are incorrect because steroid hormones do not activate cyclic AMP, G proteins, or use calcium ions as second messengers for their cellular actions. Steroid hormones act through genomic mechanisms, not via the signaling pathways associated with choices A, C, and D.

Correct Answer: A

Rationale: The correct answer is A because the binding of a steroid hormone to its receptor triggers a series of events that ultimately lead to the transcription of DNA for the gene that is 'turned on'. This process involves the hormone-receptor complex entering the nucleus, binding to specific DNA sequences, recruiting transcription factors, and initiating gene expression. Choice B is incorrect because steroid hormones do not typically use second messengers for signaling. Choice C is incorrect because while some hormones can activate enzymes indirectly, steroid hormones primarily act through gene transcription. Choice D is incorrect because cAMP is typically associated with signaling pathways involving non-steroid hormones like adrenaline.

Correct Answer: C

Rationale: Rationale: The isthmus of the thyroid gland is located between the 2nd and 3rd tracheal rings. These rings provide support to the trachea and help maintain its shape. Therefore, the correct answer is C. Summary of other choices: A: 1st and 2nd - Incorrect, as the isthmus is not related to the 1st tracheal ring. B: 1st, 2nd and 3rd - Incorrect, as the isthmus is only between the 2nd and 3rd tracheal rings. D: 2nd, 3rd and 4th - Incorrect, as the isthmus does not extend to the 4th tracheal ring.