Endocrine Glands and Hormone Classes (3A) - Bio & Biochem

Investigators studied an endocrine axis controlling plasma osmolarity. Participants received an IV infusion of a synthetic analog of a peptide hormone released from the posterior pituitary that increases water reabsorption in the kidney collecting duct. Over 60 minutes, urine flow rate decreased and urine osmolarity increased, while plasma osmolarity fell from 296 to 288 mOsm/kg. Endogenous secretion of the same pituitary hormone was inferred to decrease during the infusion. Based on the vignette, which outcome is most consistent with peptide hormone action and homeostatic feedback in this axis?
Answer: Decreased firing of hypothalamic osmoreceptors leading to reduced endogenous pituitary hormone release via negative feedback This question tests understanding of peptide hormone action and negative feedback regulation in the posterior pituitary-kidney axis controlling water balance. The synthetic analog mimics ADH (vasopressin), a peptide hormone that cannot cross cell membranes and must bind to surface receptors on kidney collecting duct cells to increase water reabsorption. The vignette shows classic ADH effects: decreased urine flow, increased urine concentration, and decreased plasma osmolarity as water is retained. When plasma osmolarity falls from 296 to 288 mOsm/kg, hypothalamic osmoreceptors detect this hypoosmotic state and reduce their firing rate, which decreases endogenous ADH release from the posterior pituitary—a negative feedback response. Choice A incorrectly describes steroid hormone action (nuclear receptors), while choice C wrongly suggests positive feedback when water homeostasis clearly involves negative feedback. To reason through hormone feedback, identify whether the hormone's effect opposes (negative) or amplifies (positive) the initial stimulus—here, water retention opposes the initial high osmolarity.
A water-soluble hormone secreted by the kidney in response to low oxygen tension was evaluated in a mouse model. Administration increased reticulocyte count over several days. In cultured bone marrow cells, the hormone caused receptor-associated kinase activation within minutes, followed by increased transcription of erythroid genes hours later. Blocking the cell-surface receptor prevented both early kinase activation and later transcriptional changes. Based on the vignette, which outcome is most consistent with this hormone class action?
Answer: Cell-surface receptor signaling that initiates intracellular kinase cascades, with delayed physiological effects due to downstream gene expression This question tests kidney hormone classification, focusing on water-soluble types promoting erythropoiesis via membrane signaling. Such hormones activate kinase cascades, leading to delayed transcriptional effects in target tissues. The vignette shows delayed reticulocyte increase with early kinase and later gene changes. Choice B is consistent as surface receptor signaling explains the timeline. Choice A fails, applying steroid diffusion to a water-soluble hormone. To reason, sequence events: immediate signaling, delayed outcomes in peptides. Block receptor to confirm membrane dependence.
A study measured responses to a hypothalamus–pituitary–thyroid axis perturbation. A peptide hormone from the anterior pituitary was administered as a bolus to healthy volunteers. Within minutes, thyroid gland secretion of a small iodinated signaling molecule increased, but maximal changes in basal metabolic rate occurred only after days. Which outcome is most consistent with the hormone classes involved and their receptor locations?
Answer: The pituitary peptide acts via membrane receptors to stimulate thyroid secretion, while the thyroid signal enters cells and alters transcription over longer timescales This question examines hormone classification in the hypothalamic-pituitary-thyroid axis, distinguishing peptide and amino acid-derived hormones by their secretion and action mechanisms. Peptide hormones like TSH from the anterior pituitary act via membrane receptors to rapidly stimulate target glands, while thyroid hormones (iodinated tyrosines) enter cells and alter transcription over days. The vignette shows quick thyroid secretion after TSH bolus but delayed metabolic rate changes, consistent with their respective fast signaling and slow genomic effects. Choice C correctly follows, as TSH stimulates thyroid hormone release, which then induces prolonged transcriptional changes in metabolism. Choice B fails by claiming thyroid hormones bind surface receptors for rapid ion channel effects, misconstruing their intracellular, transcription-based mechanism. To reason about hormone systems, differentiate classes by precursor (peptide vs. amino acid) and receptor site. Check axis dynamics by timing responses from stimulation to end-organ effects.
An adrenal gland hormone was studied during acute stress. The hormone is water-soluble, stored in secretory granules, and released rapidly after sympathetic stimulation. In target tissues, it increased heart rate and bronchodilation within seconds. A pharmacologic agent that blocks G-protein signaling eliminated most cellular responses. Based on the vignette, which outcome is most consistent with this hormone class's action?
Answer: Activation of cell-surface receptors that signal through G proteins and second messengers, producing rapid effects This question evaluates endocrine stress responses and classification of water-soluble adrenal hormones like catecholamines. These hormones are stored in granules, released rapidly, and act via membrane GPCRs with second messengers for immediate effects. The vignette describes acute sympathetic activation leading to cardiovascular changes. Choice C is consistent as rapid onset, G-protein dependence, and granule storage fit this class. Choice A fails, describing steroid mechanisms unsuitable for quick responses. To reason, identify rapid physiological shifts as GPCR-mediated. Note storage: peptides/catecholamines are vesicular, unlike steroids synthesized on demand.
Investigators examined a steroid hormone produced by the adrenal cortex that promotes sodium retention and potassium excretion in the distal nephron. During dietary sodium restriction, plasma levels of this steroid increased, and urinary sodium excretion decreased. When subjects were given a drug that blocks the upstream renal enzyme that initiates the hormone's activation cascade, plasma potassium rose and blood pressure fell. How would a disruption in adrenal cortical hormone signaling most likely affect homeostasis in this setting?
Answer: Blood pressure would fall and plasma potassium would rise because reduced steroid signaling decreases sodium reabsorption and decreases potassium excretion This question tests understanding of mineralocorticoid action and the renin-angiotensin-aldosterone system (RAAS). The steroid hormone is aldosterone from the adrenal cortex, which promotes sodium retention and potassium excretion in kidney collecting ducts, thereby increasing blood volume and pressure. The drug blocks the upstream enzyme (likely ACE or renin), preventing the cascade that stimulates aldosterone production. Without adequate aldosterone signaling, the kidney cannot retain sodium effectively (lowering blood pressure) and cannot excrete potassium normally (raising plasma potassium)—both effects described in choice C. Choice A predicts opposite effects by misunderstanding that blocking the upstream enzyme reduces (not increases) aldosterone, while choice D incorrectly identifies aldosterone as a peptide when it's clearly described as a steroid. The key principle in RAAS disruption is that reducing aldosterone activity causes reciprocal changes: sodium loss with potassium retention, manifesting as hypotension with hyperkalemia.
Researchers investigated a peptide hormone released from the posterior pituitary in response to increased plasma osmolality. In a dehydration protocol, plasma osmolality rose from 285 to 300 mOsm/kg and urine osmolality increased from 450 to 900 mOsm/kg. Administration of a selective antagonist to the hormone's receptor in the collecting duct prevented the rise in urine osmolality, and plasma osmolality remained elevated. How would a disruption in this gland's hormone signaling most directly affect homeostasis?
Answer: It would decrease the ability to concentrate urine by preventing rapid, receptor-mediated insertion of water channels into the apical membrane This question assesses endocrine gland roles and hormone classification, emphasizing peptide hormones from the posterior pituitary that enable rapid membrane changes. Peptide hormones bind GPCRs to induce second messengers, facilitating quick responses like channel insertion for water reabsorption. The scenario involves osmoregulation where the hormone counters dehydration by concentrating urine via aquaporin trafficking. Choice B follows logically as the antagonist's prevention of urine concentration and channel insertion aligns with disrupted rapid peptide signaling. Choice A fails by attributing steroid-like intracellular receptor upregulation to a rapid, peptide-mediated process, confusing classes. To reason effectively, check if blockade affects immediate membrane events (peptides) or delayed transcription (steroids). Confirm negative feedback: rising osmolality stimulates release to oppose the change, maintaining homeostasis.
Researchers compared signaling in two hormones that both regulate blood pressure. Hormone A is a peptide secreted by the heart in response to atrial stretch; Hormone B is a lipid-soluble hormone secreted by an adrenal gland region and increases Na $^+$ reabsorption in the kidney. In an experiment, Hormone A decreased Na $^+$ reabsorption within minutes, while Hormone B increased expression of Na $^+$ transport proteins over hours. Increased blood pressure reduced secretion of Hormone B. Which statement best explains the regulatory mechanism described?
Answer: Hormone B participates in negative feedback because its secretion falls when the regulated variable (blood pressure) rises This question compares endocrine regulators of blood pressure, classifying peptides and steroids by mechanisms and feedback. Peptides act rapidly via membrane receptors, steroids slowly via transcription; negative feedback reduces secretion when variables rise. The vignette contrasts atrial peptide (rapid) and adrenal steroid (delayed) in sodium handling. Choice D is correct as falling secretion with rising pressure indicates negative feedback stabilization. Choice B errs by labeling it positive feedback, misinterpreting opposition as amplification. To reason, classify by speed: fast for peptides, slow for steroids. Identify feedback: negative opposes changes, positive amplifies them.
Researchers studied an endocrine axis in which a hypothalamic peptide stimulates release of a pituitary peptide, which then stimulates a peripheral gland to secrete a lipid-soluble hormone. When peripheral hormone levels rose, both hypothalamic and pituitary peptide levels fell. In a subject with primary failure of the peripheral gland, peripheral hormone levels were low. How would a disruption in the peripheral gland function affect homeostasis?
Answer: Hypothalamic and pituitary peptide levels would be expected to increase due to loss of negative feedback This question assesses multi-level endocrine axes involving peptides and lipid-soluble hormones with feedback. Hypothalamic and pituitary peptides drive peripheral steroids, which negatively feedback to suppress upstream release. The vignette outlines an axis where rising peripheral hormone lowers upstream peptides. Choice A follows as peripheral failure reduces feedback, increasing both peptides. Choice B is incorrect, predicting decreased levels from enhanced feedback. For reasoning, map axis: low peripheral lifts inhibition upstream. Note consistency: primary failure lowers end hormone, elevates tropins.
A physiology lab compares two endocrine signals: Hormone X is synthesized from cholesterol in the adrenal cortex and circulates largely bound to plasma proteins; Hormone Y is synthesized as a preprohormone in the anterior pituitary and circulates freely in plasma. After acute secretion, target cells respond to X with changes in gene expression that peak after several hours, whereas target cells respond to Y within minutes by altering phosphorylation states of existing enzymes. Based on the vignette, which statement best explains the mechanism of action for Hormone X? (Assume both hormones participate in negative feedback at the hypothalamus/pituitary axis.)
Answer: Hormone X diffuses across the plasma membrane to bind an intracellular receptor that functions as a transcription factor This question tests the ability to distinguish between steroid and peptide hormone mechanisms based on their chemical properties and cellular responses. Steroid hormones are lipophilic molecules derived from cholesterol that can diffuse across plasma membranes to bind intracellular receptors, which then function as transcription factors to alter gene expression over hours. The vignette describes Hormone X as cholesterol-derived, protein-bound in circulation, and causing delayed gene expression changes - all classic features of steroid hormones like cortisol or aldosterone from the adrenal cortex. The correct answer C accurately describes steroid hormone action: membrane diffusion and intracellular receptor binding that regulates transcription. Answer A incorrectly attributes G-protein/cAMP signaling to a steroid, when this mechanism is typical of peptide hormones that cannot cross membranes. When distinguishing hormone classes, remember that synthesis from cholesterol, extensive protein binding, and delayed transcriptional effects are hallmarks of steroid hormones, while rapid phosphorylation cascades indicate peptide hormone action.
An experiment compared two synthetic hormones: Compound X is hydrophilic and remains largely in plasma; Compound Y is lipophilic and partitions into cell membranes. In target cells, X produced a rapid increase in intracellular $\mathrm{Ca^{2+}}$ within seconds, while Y increased expression of a transporter protein after 6 hours. Based on the vignette, which pairing of receptor location and primary signaling mode is most consistent with Compounds X and Y?
Answer: X: cell-surface receptor coupled to second messengers; Y: intracellular receptor that modulates transcription This question evaluates hormone classification by solubility and receptor mechanisms in endocrine signaling. Hydrophilic hormones (like peptides) bind membrane receptors for rapid second-messenger effects, while lipophilic hormones (like steroids) bind intracellular receptors for transcriptional changes. The vignette contrasts Compound X's extracellular retention and fast Ca2+ rise with Y's membrane partitioning and delayed transporter expression. Choice B matches, linking X to surface receptors with second messengers and Y to intracellular transcription modulation. Choice A fails by reversing assignments, misconstruing hydrophilic signals as intracellular and ignoring solubility's role in receptor access. For hormone systems, use solubility to deduce receptor location and primary signaling mode. Confirm by matching timelines: rapid for membrane pathways, delayed for genomic effects.
Investigators examined a lipid-soluble hormone secreted by an endocrine gland located atop the kidneys. In healthy volunteers, an intravenous infusion of this hormone for 30 minutes increased expression of epithelial Na $^+$ channels in distal nephron cells and was followed by a gradual rise in arterial pressure over 2 hours. When arterial pressure rose, secretion of the hormone decreased. A separate arm used a competitive antagonist that prevented the hormone–receptor complex from binding DNA response elements. Plasma electrolyte data are shown: Condition | Plasma [Na $^+$ ] (mM) | Plasma [K $^+$ ] (mM) Baseline | 140 | 4.2 Infusion | 143 | 3.6 Infusion + DNA-binding antagonist | 140 | 4.2 Based on the vignette, which outcome is most consistent with this hormone class's mechanism of action?
Answer: Increased transcription of transport proteins following intracellular receptor binding, with effects blunted by blocking hormone–DNA interactions This question tests understanding of endocrine gland functions and hormone classification, specifically distinguishing steroid hormones from peptides based on solubility and signaling mechanisms. Steroid hormones are lipid-soluble, diffuse into cells, and bind intracellular receptors to regulate gene transcription, producing delayed effects. In this vignette, the hormone from the adrenal cortex modulates renal sodium handling and blood pressure via transcriptional changes in transport proteins. Choice B logically follows because the delayed rise in pressure, increased channel expression, and blunting by a DNA-binding antagonist align with steroid mechanisms requiring gene regulation. Choice A fails as it describes rapid peptide hormone action via GPCRs, which contradicts the lipid solubility and gradual effects observed. To reason about hormone systems, evaluate onset time: rapid effects suggest membrane receptors typical of peptides, while delayed transcriptional changes indicate steroids. Additionally, confirm feedback type—negative feedback, as here, stabilizes variables by opposing changes.
A peptide hormone secreted by adipose tissue signals energy sufficiency to the hypothalamus. In a study, chronic elevation of the hormone did not reduce food intake or body mass in a subset of participants, despite high circulating hormone levels. Neurons from these participants showed reduced phosphorylation of a downstream signaling protein after hormone exposure. Based on the vignette, which outcome is most consistent with disrupted hormone signaling?
Answer: Receptor or post-receptor resistance that blunts intracellular signaling despite high hormone levels, impairing homeostatic regulation of intake This question tests adipose peptide functions in energy homeostasis and signaling disruptions. Peptides bind hypothalamic receptors for phosphorylation-mediated appetite control; resistance blunts responses. The vignette describes high hormone without intake reduction, with reduced phosphorylation. Choice B aligns as resistance impairs signaling despite levels. Choice A fails, suggesting hypersensitivity, opposite to blunted effects. For reasoning, correlate levels and effects: high without response indicates resistance. Examine downstream: reduced signaling confirms post-receptor defect.
A patient has a pituitary adenoma that secretes high levels of a tropic peptide hormone that stimulates an endocrine gland to produce a cholesterol-derived hormone. Despite elevated circulating levels of the downstream steroid, the pituitary peptide remains high. Which explanation best accounts for the failure of homeostatic regulation?
Answer: Autonomous tumor secretion overrides normal negative feedback, so elevated downstream steroid fails to suppress pituitary output This question assesses feedback dysregulation in pituitary adenomas affecting peptide and steroid hormone axes. Tropic peptide hormones stimulate downstream glands, with negative feedback from steroids suppressing pituitary secretion to maintain homeostasis. The vignette describes high ACTH (peptide) from adenoma driving elevated cortisol (steroid), yet persistent high ACTH due to tumor autonomy. Choice B explains this as the tumor overriding feedback, preventing suppression despite high steroid levels. Choice C fails by claiming steroids cannot bind surface receptors for feedback, misconstruing intracellular signaling in pituitary cells. In hormone systems, identify autonomous secretion by checking if elevated downstream signals fail to inhibit upstream output. Evaluate regulation by distinguishing negative from positive feedback in endocrine loops.
A lipid-soluble hormone produced by ovarian tissue was assessed during a controlled cycle study. When circulating levels of the hormone were experimentally held constant at a moderate level, secretion of an upstream pituitary peptide decreased. However, when the hormone was raised rapidly to a high level for a brief window, upstream pituitary peptide secretion increased instead. Which statement best explains the regulatory mechanism described?
Answer: The hormone can participate in context-dependent feedback, with negative feedback at moderate levels and positive feedback at high levels This question evaluates ovarian steroid regulation, classifying feedback as context-dependent. Steroids can switch feedback polarity based on levels, negative at moderate, positive at high for surges. The vignette shows decreased pituitary at moderate, increased at high hormone. Choice D is consistent as context-dependent feedback explains the switch. Choice B errs, tying feedback switch to peptides only. To reason, test levels: moderate suppresses, peak amplifies in positive loops. Identify contexts: reproductive cycles often feature positive feedback for events like ovulation.
Researchers studied a lipophilic hormone class secreted by the adrenal cortex (zona fasciculata) during an acute stress protocol. Plasma hormone increased within 15 minutes, followed by increased hepatic expression of a gluconeogenic enzyme mRNA at 60 minutes. Subjects then received an infusion of a synthetic analog that readily crosses membranes. Selected observations: - Cytosolic receptor abundance decreased after infusion - Nuclear receptor–DNA complexes increased - Plasma upstream pituitary signal decreased over 2 hours Based on the vignette, which outcome is most consistent with this hormone class's mechanism of action and feedback regulation?
Answer: Diffusion into target cells followed by receptor binding and altered gene transcription, with negative feedback reducing upstream pituitary signaling This question tests the understanding of steroid hormone mechanisms in the endocrine system, focusing on their classification as lipophilic signals from glands like the adrenal cortex and their role in stress responses. Steroid hormones, derived from cholesterol, are lipophilic and can diffuse across cell membranes to bind intracellular receptors, ultimately altering gene transcription. In this vignette, the hormone from the zona fasciculata is likely a glucocorticoid like cortisol, which increases during stress and induces gluconeogenic enzyme expression over minutes to hours. The observations of decreased cytosolic receptors, increased nuclear receptor-DNA complexes, and reduced upstream pituitary signaling (ACTH) logically follow from the steroid's mechanism of nuclear translocation and negative feedback on the hypothalamic-pituitary-adrenal axis, as described in choice B. A distractor like choice A fails by describing a peptide hormone mechanism involving rapid GPCR signaling and second messengers, which would not involve transcriptional changes or the observed receptor dynamics, misconstruing the hormone class's solubility and action timeline. To reason about hormone systems, always classify hormones by solubility to predict receptor location and onset of effects. Additionally, evaluate feedback loops by checking if downstream signals inhibit upstream regulators to maintain homeostasis.
A peptide hormone secreted by the thyroid was administered to healthy subjects. Serum calcium decreased modestly over 1 hour, and osteoclast activity markers decreased. The hormone's effects were blocked by a drug that prevents activation of a membrane-associated G protein. Endogenous secretion of the hormone increased when serum calcium was experimentally raised. Based on the vignette, which outcome is most consistent with peptide hormone action?
Answer: The hormone binds cell-surface receptors and triggers second messengers to rapidly alter osteoclast activity This question tests thyroid peptide functions in calcium homeostasis and classification via rapid signaling. Peptides bind membrane receptors, using second messengers for quick cellular modulation like inhibiting bone resorption. Here, the hormone lowers calcium by reducing osteoclast activity, with G-protein involvement. Choice B aligns as rapid effects and G-protein blockade fit peptide mechanisms. Choice A is incorrect, applying steroid nuclear binding to a rapid process. For reasoning, associate immediate ion changes with membrane signaling. Confirm feedback: secretion rises with high calcium, negatively feeding back to lower it.
Researchers investigated a peptide hormone secreted by pancreatic islet cells in response to rising plasma glucose. In isolated hepatocytes, the hormone increased phosphorylation of a key metabolic enzyme within 2 minutes. In a separate experiment, a toxin that blocks vesicular exocytosis in endocrine cells prevented the hormone's rise after a glucose load. Based on the vignette, which statement best explains the hormone's secretion and action mechanism?
Answer: The hormone is stored in secretory granules and released by regulated exocytosis, then binds a membrane receptor to trigger rapid second-messenger signaling This question assesses peptide hormone classification and function in pancreatic endocrine signaling, emphasizing secretion and rapid metabolic effects. Peptide hormones like insulin are synthesized in the rough ER, stored in granules, and released by exocytosis to act via membrane receptors and second messengers. The vignette details insulin's response to glucose, quick enzyme phosphorylation in hepatocytes, and exocytosis blockade preventing secretion. Choice B aligns, as it describes regulated exocytosis and fast membrane receptor signaling for metabolic control. Choice C fails by attributing direct DNA binding to peptides, a misconception confusing them with steroids that act intracellularly. For hormone systems, classify by synthesis site to predict storage and release modes. Verify mechanisms by correlating action speed with receptor location and signaling pathways.
A study examined a peptide hormone secreted by pancreatic islet cells in response to elevated plasma glucose. In isolated hepatocytes, adding the hormone increased phosphorylation of a cytosolic kinase within 2 minutes and increased glycogen synthesis by 20 minutes. In vivo, infusion of the hormone lowered plasma glucose; subsequently, secretion of the hormone from the pancreas decreased. Which statement best explains the mechanism of action for this hormone and the observed regulation?
Answer: The hormone binds a cell-surface receptor to initiate a second-messenger cascade, and the resulting fall in glucose reduces further hormone secretion via negative feedback This question tests understanding of peptide hormone signaling and glucose homeostasis feedback loops, specifically describing insulin action. The hormone's peptide nature means it cannot cross cell membranes and must bind surface receptors to initiate rapid intracellular signaling—here, phosphorylating a kinase within 2 minutes that promotes glycogen synthesis. The vignette demonstrates classic negative feedback: elevated glucose triggers insulin secretion, insulin lowers glucose by promoting storage, and the resulting glucose decrease reduces further insulin secretion from pancreatic beta cells. Choice A incorrectly describes steroid hormone action with nuclear receptors and impossibly fast transcriptional effects, while choice D wrongly invokes positive feedback when glucose regulation clearly requires negative feedback to prevent hypoglycemia. To identify feedback type in metabolic regulation, determine if the hormone's effect (lowering glucose) opposes the initial stimulus (high glucose), indicating negative feedback that maintains homeostasis.
During a dehydration challenge, an endocrine signal released from the posterior pituitary increases water reabsorption in the kidney collecting duct within minutes. In isolated collecting duct cells, the signal increases intracellular cAMP and causes insertion of water channels into the apical membrane. When plasma osmolality returns toward baseline, secretion of the posterior pituitary signal decreases. Based on the vignette, which statement best explains the mechanism of action for this hormone class? (Assume normal renal perfusion and intact hypothalamic osmoreceptors.)
Answer: The hormone acts through a cell-surface receptor and second messenger to rapidly change membrane protein trafficking, with negative feedback as osmolality normalizes This question tests understanding of posterior pituitary peptide hormone signaling, specifically vasopressin (ADH) action in osmoregulation. Peptide hormones from the posterior pituitary, like ADH, are water-soluble molecules that bind cell-surface receptors and activate second messenger systems for rapid physiological responses. The vignette describes classic ADH action: rapid water reabsorption via cAMP-mediated aquaporin insertion in collecting duct cells, with negative feedback as osmolality normalizes - when plasma becomes less concentrated, osmoreceptors reduce ADH release. The correct answer B accurately describes this mechanism: cell-surface receptor binding, second messenger signaling for rapid membrane protein trafficking, and negative feedback regulation. Answer A incorrectly suggests nuclear DNA binding and transcriptional effects, which would be characteristic of steroid hormones and take hours, not minutes. A key principle for posterior pituitary hormones is that they produce rapid effects through surface receptors and protein trafficking, with tight negative feedback control maintaining homeostasis of critical parameters like osmolality and blood pressure.
An experiment evaluated a lipid-soluble hormone synthesized in the gonads from a cholesterol-derived precursor. Target cells showed increased transcription of a differentiation marker after 6 hours of hormone exposure. When a competitive antagonist that cannot cross the plasma membrane was added to the extracellular medium, the hormone's effect on transcription persisted. Which statement best explains the mechanism of action for this hormone?
Answer: The hormone binds an intracellular receptor, so an extracellular antagonist that cannot enter cells will not block receptor binding and transcriptional effects This question tests understanding of steroid hormone mechanism of action based on experimental antagonist results. The hormone's lipid solubility and cholesterol derivation identify it as a steroid (likely testosterone or estrogen from gonads), which must cross the plasma membrane to bind intracellular receptors and alter gene transcription—explaining the 6-hour delay for transcriptional effects. The critical experimental finding is that an extracellular antagonist fails to block the hormone's action, proving the receptor must be intracellular since the membrane-impermeant antagonist cannot reach it. Choice A incorrectly suggests the hormone uses surface receptors, contradicting both its lipid solubility and the antagonist results, while choice C wrongly attributes steroid properties (vesicular storage) to this lipid-soluble hormone. The diagnostic principle for identifying intracellular receptors is that only membrane-permeant antagonists can block their activation, as extracellular compounds cannot access the cytoplasmic or nuclear binding sites.

Start this quiz