Which of the following is the most common cause of hypothyroidism worldwide?

Correct Answer: D

Rationale: Iodine deficiency is the most common cause of hypothyroidism worldwide. Iodine is an essential mineral required for the production of thyroid hormones. Without sufficient iodine, the thyroid gland is unable to produce an adequate amount of thyroid hormones, leading to hypothyroidism. Areas with low iodine levels in the soil or water are more prone to iodine deficiency, which can result in a range of thyroid disorders, including hypothyroidism. Efforts to prevent iodine deficiency include iodized salt programs and dietary interventions to ensure an adequate intake of iodine.

Correct Answer: C

Rationale: The prolonged ingestion of iodine causing goiter is known as the Wolf-Chaikoff effect. This phenomenon occurs when high levels of iodine cause a temporary inhibition of thyroid hormone synthesis, resulting in a decrease in thyroid hormone production. In some individuals, this can lead to the development of a goiter due to the lack of thyroid hormone feedback regulation. It is important to note that the Wolf-Chaikoff effect typically resolves on its own as the thyroid gland adapts to the increased iodine levels over time.

Correct Answer: A

Rationale: Addison's disease, also known as primary adrenal insufficiency, is a condition where the adrenal glands do not produce enough steroid hormones, mainly cortisol and sometimes aldosterone. This deficiency in steroid hormone production is typically caused by autoimmune destruction of the adrenal cortex. Without enough cortisol and aldosterone, various symptoms may arise, such as fatigue, weight loss, low blood pressure, and skin discoloration. Therefore, option A accurately describes the nature of Addison's disease.

Correct Answer: C

Rationale: Hypercalcemia refers to elevated levels of calcium in the blood. The effects of hypercalcemia include increased urinary frequency (polyuria), gastrointestinal symptoms such as constipation rather than diarrhea, kidney stones (nephrolithiasis), and various cardiac abnormalities. One key cardiac abnormality associated with hypercalcemia is a prolonged QT interval rather than a shortened QT interval. Prolongation of the QT interval can predispose individuals to arrhythmias, particularly Torsades de pointes. Therefore, the correct answer is C, as a shortened QT interval is not a typical effect of hypercalcemia.

Correct Answer: A

Rationale: In the syndrome of inappropriate antidiuretic hormone (SIADH), there is excessive secretion of antidiuretic hormone (ADH) leading to increased water reabsorption in the kidney tubules. This results in dilutional hyponatremia because the amount of water reabsorbed is greater than the electrolytes, such as sodium, leading to a relative decrease in their concentration in the blood. As a result, hyponatremia occurs in SIADH, contributing to symptoms such as nausea, confusion, and seizures due to the imbalance in electrolytes.

Correct Answer: B

Rationale: Small cell (oat cell) carcinoma of the lung is most commonly associated with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). This paraneoplastic syndrome occurs in patients with small cell lung cancer due to the production of antidiuretic hormone (ADH) by the tumor cells. The excessive release of ADH leads to water retention and dilutional hyponatremia, causing symptoms such as nausea, confusion, seizures, and potentially life-threatening complications. Other types of lung cancer, such as squamous cell carcinoma, large cell carcinoma, and adenocarcinoma, are less commonly associated with SIADH.

Correct Answer: C

Rationale: Hormones are chemical messengers that are produced by various glands in the body and released into the bloodstream to signal and regulate the activity of specific target cells. Unlike energy sources that can be metabolized for energy production (like carbohydrates and fats), hormones are not typically used as an energy source themselves (Choice B). Hormones are typically not biologically inert by themselves (Choice A), as they have specific functions in regulating various physiological processes in the body. While some hormones may be stored in secretory granules before release, not all hormones are stored in this manner (Choice D). Additionally, hormones are not always released by exocytosis (Choice E), as some hormones can also be released through other mechanisms such as diffusion. The correct choice, therefore, is C: hormones are incorporated as a structural moiety into another molecule, as they bind to specific receptors on target cells to elicit a cellular response and regulate physiological functions.

Correct Answer: D

Rationale: Vanillylmandelic acid (VMA) is a metabolite of catecholamines, such as epinephrine and norepinephrine. Phaeochromocytoma is a catecholamine-secreting tumor that arises from the chromaffin cells in the adrenal medulla or sympathetic ganglia. As a result of the increased production of catecholamines in patients with phaeochromocytoma, there is an increase in VMA excretion in the urine. This increase in VMA excretion is a significant diagnostic feature of phaeochromocytoma and helps in its detection through urine testing. Therefore, the correct choice is D. Phaeochromocytoma.

Correct Answer: D

Rationale: Growth hormone is primarily released during sleep, particularly during the first few hours of deep sleep. This release is pulsatile in nature, with the majority occurring at night. Growth hormone plays a critical role in growth, repair, and development of body tissues during periods of rest and sleep. Additionally, the secretion of growth hormone is also influenced by factors such as stress, exercise, and nutritional status.