Which of the following hormones acts on its target tissues by a steroid hormone mechanism of action?

Correct Answer: A

Rationale: The correct answer is A: Thyroid hormone. Thyroid hormone acts on its target tissues by a steroid hormone mechanism of action because it is a lipid-soluble hormone that can diffuse through cell membranes and bind to intracellular receptors. These hormone-receptor complexes then directly regulate gene expression. In contrast, choices B, C, and D do not follow a steroid hormone mechanism of action. Parathyroid hormone (PTH) acts through G-protein coupled receptors, Antidiuretic hormone (ADH) acts through V2 receptors, and β1 adrenergic agonists act through beta-adrenergic receptors, which are all distinct from the mechanism of action of steroid hormones.

Correct Answer: C

Rationale: The correct answer is C: G protein acts as the link between first and second messengers. Rationale: 1. Peptide hormones cannot enter the cell membrane due to their large size. 2. Hormones do not directly stimulate genes; they act through signaling pathways. 3. G proteins are key in transmitting signals from hormone receptors to second messengers, facilitating cellular responses. 4. The hormone receptor complex typically stays on the cell membrane to initiate signaling cascades.

Correct Answer: A

Rationale: The correct answer is A because cellular responses to hormones initiating second-messenger systems can activate various pathways. Different hormones can utilize different second messengers such as cAMP, cGMP, IP3, and DAG. This diversity allows for a wide range of responses in target cells. Choice B is incorrect because cyclic AMP phosphodiesterase degrades cAMP, not forms an active second messenger. Choice C is incorrect as it describes the mechanism of a specific protein kinase, which is not a characteristic of second-messenger systems. Choice D is incorrect since hormone binding to intracellular receptors typically leads to gene transcription regulation, not direct activation of second-messenger systems.

Correct Answer: B

Rationale: The correct answer is B: Calcium. Calcium ions are commonly used as second messengers for amino acid-base hormones due to their ability to trigger various cellular responses. When the hormone binds to its receptor on the cell surface, it activates specific signaling pathways that lead to an increase in intracellular calcium levels. This rise in calcium ions then initiates downstream signaling cascades, ultimately regulating processes such as gene expression, cell growth, and metabolism. Choice A - Iron: Iron is not typically involved in signaling pathways for amino acid-base hormones. It is more commonly known for its role in oxygen transport and storage in hemoglobin. Choice C - Sodium: While sodium ions play essential roles in cellular functions, they are not typically utilized as second messengers for amino acid-base hormones. Sodium is more commonly associated with maintaining cell membrane potential and fluid balance. Choice D - Chlorine: Chlorine ions are not typically involved in signaling pathways for amino acid-base hormones. Chlorine is primarily known for its role in maintaining

Correct Answer: C

Rationale: The correct answer is C: Pancreas. The pancreas is an endocrine gland located in the abdominal cavity near the small intestine. It secretes hormones like insulin and glucagon that regulate blood sugar levels. The adrenal gland (A) is located on top of the kidneys, the hypothalamus (B) is in the brain, and the thyroid gland (D) is located in the neck. These glands are not near the small intestine, making them incorrect choices.