Which of the following statements correctly describes the pathophysiology of Cushing's disease?

Correct Answer: A

Rationale: Rationale: A: Correct. Cushing's disease is caused by a pituitary adenoma secreting excess ACTH, stimulating adrenal cortex to produce excess cortisol. B: Incorrect. Cushing's disease does not result from excess cortisol secretion by the pituitary adenoma. C: Incorrect. Cushing's disease is not caused by an adrenal adenoma secreting excess ACTH. D: Incorrect. Cushing's disease is not caused by an adrenal adenoma secreting excess cortisol. Summary: The correct answer is A because it accurately describes the pathophysiology of Cushing's disease, where a pituitary adenoma secretes excess ACTH leading to increased cortisol production. Choices B, C, and D are incorrect as they misinterpret the primary source of hormone overproduction in Cushing's disease.

Correct Answer: D

Rationale: Step-by-step rationale: 1. Gynaecomastia is a known side effect of spironolactone, digitalis, and cimetidine. 2. Rifampicin is not associated with gynaecomastia. 3. Rifampicin is an antibiotic used to treat tuberculosis and other infections. 4. The mechanism of action of rifampicin does not involve hormonal changes that could lead to gynaecomastia. Summary of incorrect choices: - A: Spironolactone is a potassium-sparing diuretic that can cause gynaecomastia. - B: Digitalis (digoxin) can lead to gynaecomastia as a side effect. - C: Cimetidine, an H2 receptor antagonist, is also known to cause gynaecomastia.

Correct Answer: D

Rationale: The correct answer is D because synthesis of thyroid hormones (T3 and T4) occurs by the coupling of iodinated tyrosine residues on thyroglobulin within thyroid follicles. Thyroglobulin acts as a precursor for thyroid hormone synthesis. Choices A, B, and C are incorrect because thyroid hormone synthesis does not occur in parafollicular cells, extracellularly in plasma, or require cholesterol for the coupling of iodinated tyrosine residues on thyroglobulin.

Correct Answer: C

Rationale: Sertoli cells are responsible for producing and secreting Androgen binding globulin (ABG). They play a crucial role in supporting spermatogenesis by providing structural and nutritional support to developing sperm cells. ABG helps in the transport of testosterone and other androgens within the seminiferous tubules, aiding in the regulation of spermatogenesis. Testosterone is primarily produced by Leydig cells in the testes, not by Sertoli cells, making option A incorrect. Option B, Oestrogen, is mainly synthesized in the ovaries, while Sertoli cells do not produce GnRH (option D), which is secreted by the hypothalamus. Therefore, the correct answer is C, as Sertoli cells are primarily responsible for producing and secreting Androgen binding globulin.

Correct Answer: D

Rationale: The correct answer is D because the anterior pituitary is regulated by hypothalamic neuropeptides. The hypothalamus secretes releasing and inhibiting hormones that control the anterior pituitary's hormone release. This close interaction between the hypothalamus and the anterior pituitary is known as the hypothalamic-pituitary axis. A is incorrect because the anterior pituitary is not neural tissue, it is an endocrine gland. B is incorrect because the anterior pituitary synthesizes and secretes peptide hormones, not steroid hormones. C is incorrect because the anterior pituitary is located at the base of the brain, not above the kidney.

Correct Answer: A

Rationale: The correct answer is A because parathyroid hormone does not directly stimulate osteoblasts to increase bone formation. Instead, it indirectly increases bone resorption by stimulating osteoclasts. Parathyroid hormone also increases calcium reabsorption from the distal tubule of the kidney (choice C) and inhibits phosphate reabsorption in the proximal tubule of the kidney (choice D). By understanding the physiological actions of parathyroid hormone, we can deduce that direct stimulation of osteoblasts is not one of its functions.

Correct Answer: A

Rationale: The correct answer is A: Cirrhosis. In cirrhosis, there is a decrease in the binding proteins for T4, leading to decreased total T4 levels in the plasma. Pregnancy, euthyroid sick syndrome, and familial excess thyroid binding globulin are all conditions associated with increased total T4 levels due to various physiological changes. In pregnancy, there is an increase in thyroid hormone production to support fetal development. Euthyroid sick syndrome is a condition where there is a decrease in binding proteins, leading to an increase in free T4 levels but normal total T4 levels. Familial excess thyroid binding globulin causes an increase in total T4 levels due to excessive binding proteins.

Correct Answer: C

Rationale: Diabetes insipidus is a condition where the kidneys are unable to conserve water, leading to the excretion of large volumes of dilute urine. This is due to either a lack of antidiuretic hormone (ADH) production (central DI) or the kidneys' insensitivity to ADH (nephrogenic DI). Choice A is incorrect as the urine is dilute, not concentrated. Choice B is incorrect as diabetes insipidus results in large, not small, urine volumes. Choice D is incorrect as the urine is dilute, not concentrated. Therefore, choice C accurately describes diabetes insipidus.

Correct Answer: C

Rationale: Rationale: 1. GnRH directly binds to pituitary receptors. 2. GnRH specifically targets gonadotroph cells to release LH and FSH. 3. CRH and ACTH are related to stress and adrenal function, not reproductive hormones. 4. Tremor is unrelated to pituitary hormone regulation. Summary: Gonadotrophin releasing hormone (GnRH) is the correct answer as it directly stimulates the release of LH and FSH from the pituitary gland. Other choices are incorrect as they are not involved in the regulation of reproductive hormones.