Steroid hormones exert their action by ________.

Correct Answer: A

Rationale: The correct answer is A because steroid hormones are lipid-soluble molecules that can pass through the cell membrane and bind to intracellular receptors in the nucleus. This binding activates or inhibits gene transcription, leading to changes in protein synthesis and cellular responses. Choice B is incorrect because cAMP activity is typically associated with peptide hormones, not steroid hormones. Choice C is incorrect because steroid hormones do not stimulate glycogen synthesis. Choice D is incorrect because steroid hormones do not directly affect blood pressure regulation.

Correct Answer: B

Rationale: The correct answer is B: Thyroxine does not require a second messenger to effect a response. Thyroxine is a non-peptide hormone derived from the amino acid tyrosine, not a peptide hormone. It acts directly on the cell by binding to nuclear receptors, which then bind to specific regions of DNA to regulate gene expression. This mechanism is different from peptide hormones, which typically require second messengers like cAMP or calcium to transmit their signal inside the cell. Therefore, choice B is correct. Choices A, C, and D are incorrect because thyroxine does not cause positive feedback, is not highly specific in cell targeting, and does not target all cells for cellular metabolism stimulation.

Correct Answer: B

Rationale: Step 1: Negative feedback is the primary mechanism in endocrine system regulation to maintain homeostasis. Step 2: When hormone levels reach a certain set point, negative feedback signals the gland to stop producing that hormone. Step 3: This ensures that hormone levels remain within a specific range. Step 4: Positive feedback would lead to further hormone secretion, disrupting homeostasis. Step 5: Neural feedback is not a common mechanism for regulating endocrine secretions.

Correct Answer: D

Rationale: The correct answer is D: ovary. The anterior pituitary gland secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that regulate the function of the ovaries. FSH stimulates the growth of ovarian follicles, while LH triggers ovulation and stimulates the production of progesterone and estrogen. The pancreas (A) is mainly controlled by insulin and glucagon from the pancreas itself. The parathyroid gland (B) is regulated by parathyroid hormone (PTH) secreted by the parathyroid glands. The adrenal medulla (C) is controlled by the sympathetic nervous system, not by anterior pituitary hormones.

Correct Answer: B

Rationale: The correct answer is B because parathyroid hormone (PTH) actually increases blood calcium levels. PTH achieves this by releasing calcium from bone (A), promoting reabsorption of calcium in the kidneys (C), and activating Vitamin D for increased calcium absorption in the intestines (D). By decreasing blood calcium levels, it would go against the primary function of PTH. This choice is incorrect as PTH is known for its role in increasing blood calcium levels through the mentioned mechanisms.