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: C

Rationale: The correct answer is C: Myxoedema. Myxoedema is associated with hypothyroidism, not SIADH. Guillain-Barre syndrome, subacute bacterial endocarditis, and bronchogenic carcinoma are all conditions that can lead to SIADH due to various mechanisms. Guillain-Barre syndrome can cause SIADH through autonomic dysfunction, endocarditis through the release of inflammatory cytokines, and bronchogenic carcinoma through production of ectopic ADH. Therefore, myxoedema is the only incorrect choice as it does not typically lead to SIADH.

Correct Answer: C

Rationale: The correct answer is C because dipsogenic diabetes insipidus is caused by a defect or damage to the hypothalamus, not the pituitary gland or pituitary stalk. The hypothalamus regulates thirst and water intake, so damage to this area can lead to excessive thirst and dilute urine output characteristic of diabetes insipidus. Choices A and D incorrectly attribute the cause to the pituitary gland or pituitary stalk, which are not directly involved in regulating thirst. Choice B is incorrect as it mentions malfunction of the hunger mechanism, which is not relevant to dipsogenic diabetes insipidus. Thus, choice C is the most accurate definition based on the pathophysiology of the condition.

Correct Answer: C

Rationale: The correct answer is C: Glucose-6-phosphatase deficiency. This enzyme is essential for gluconeogenesis and glycogenolysis, so its deficiency leads to fasting hypoglycemia. Galactosaemia (A) can cause hypoglycemia due to impaired galactose metabolism. Insulinoma (B) results in excess insulin production causing hypoglycemia. Systemic carnitine deficiency (D) can lead to hypoglycemia by impairing fatty acid oxidation, but it does not directly cause fasting hypoglycemia as seen in Glucose-6-phosphatase deficiency.

Correct Answer: B

Rationale: The correct answer is B: Cretinism. Cretinism is the most common cause of preventable mental deficiency worldwide due to severe iodine deficiency during pregnancy. Iodine is crucial for thyroid hormone production, which is essential for fetal brain development. Beriberi disease (A) is caused by thiamine deficiency and affects the nervous system, not mental deficiency. Folate deficiency (C) can cause neural tube defects but not mental deficiency. Vitamin A deficiency (D) can lead to blindness and immune system issues, but not the most common cause of preventable mental deficiency.

Correct Answer: B

Rationale: The correct answer is B: Intranasal vasopressin may be helpful. In diabetes insipidus, there is a deficiency of vasopressin leading to excessive urination and thirst. Administering vasopressin intranasally can help replace the deficient hormone, reducing urine output and thirst. Hypernatremia (choice A) can occur due to excessive water loss, but it is not always present. There is no osmolar gap (choice C) in diabetes insipidus. Urine specific gravity (choice D) is typically low in diabetes insipidus due to the dilute urine produced.

Correct Answer: B

Rationale: The correct answer is B: Cortisol. The adrenal cortex produces cortisol, a steroid hormone involved in stress response and metabolism. Norepinephrine (A) is produced by the adrenal medulla. 5-DHEA (C) is a precursor to sex hormones and not directly produced by the adrenal cortex. Aldosterone (D) is produced by the adrenal cortex but is involved in regulating salt and water balance, not directly related to cortisol production.

Correct Answer: B

Rationale: The correct answer is B: Weight loss. Hypothyroidism is characterized by an underactive thyroid gland, leading to a decreased metabolic rate. This typically results in weight gain, not weight loss. Choices A, C, and D are all associated with hypothyroidism. Loss of libido can occur due to hormonal imbalances, cardiac failure can result from the decreased metabolic rate affecting the heart, and organic psychosis can be a symptom of severe untreated hypothyroidism.

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.