The element essential for normal thyroid function is:

Correct Answer: C

Rationale: The correct answer is C: Iodine. Iodine is essential for the production of thyroid hormones T3 and T4. These hormones regulate metabolism, growth, and development. Potassium (A), Sodium (B), and Calcium (D) do not play a direct role in thyroid function. Potassium and Sodium are electrolytes important for nerve function and fluid balance, while Calcium is crucial for bone health and muscle function. Therefore, the correct answer is Iodine as it is specifically required for normal thyroid function.

Correct Answer: B

Rationale: The correct answer is B: Weight gain. Addison's disease is characterized by adrenal insufficiency, leading to symptoms such as salt craving, hypoglycemia, and hyperpigmentation due to increased melanin production. Weight loss, not weight gain, is commonly associated with Addison's disease due to decreased cortisol levels. Salt craving is due to sodium loss, hypoglycemia results from insufficient cortisol levels, and hyperpigmentation is caused by elevated ACTH levels stimulating melanin production. Weight gain is not a typical symptom, making it the correct answer.

Correct Answer: D

Rationale: The correct answer is D, receptor-mediated stimulus. Hormones can be stimulated by hormonal, humoral, or neural factors, but not directly by receptors. Hormonal stimulus involves other hormones triggering hormone release, humoral stimulus involves changing blood levels of ions or nutrients triggering hormone release, and neural stimulus involves direct nerve stimulation triggering hormone release. Receptor-mediated stimulus is not a recognized type of hormone stimulus because hormones do not directly respond to receptors in this context.

Correct Answer: A

Rationale: Aldosterone is the correct answer because it specifically acts on the kidneys to increase the reabsorption of sodium. It is produced by the adrenal glands in response to low blood pressure or low sodium levels. Aldosterone binds to receptors in the kidneys, leading to increased sodium reabsorption and potassium excretion. Angiotensin plays a role in stimulating the release of aldosterone, but it does not directly affect sodium reabsorption. ADH regulates water reabsorption, not sodium. ACTH stimulates the adrenal glands to produce cortisol, not aldosterone. Therefore, aldosterone is the hormone responsible for the reabsorption of sodium in the kidneys.

Correct Answer: B

Rationale: The correct answer is B because free fatty acid mobilization is primarily regulated by cortisol, not dehydroepiandrosterone. Cortisol promotes lipolysis and stimulates the release of free fatty acids from adipose tissue. Dehydroepiandrosterone is a precursor hormone mainly involved in sex hormone production and has minimal effects on free fatty acid mobilization. Choices A, C, and D are correctly paired as gluconeogenesis is stimulated by cortisol, muscle glycogenolysis is regulated by epinephrine, and kaliuresis is induced by aldosterone.