Assertion (A): Aldosterone secretion increases in response to high blood volume. Reason (R): Activation of the renin-angiotensin system stimulates aldosterone release from the adrenal cortex.

Here are the answers to the assertion-reason questions:

1. Assertion (A): Plasma cortisol levels are decreased in secondary adrenal insufficiency.
Reason (R): Secondary adrenal insufficiency results from pituitary failure leading to reduced ACTH secretion.

• Assertion (A): Secondary adrenal insufficiency is caused by a lack of ACTH from the pituitary gland, which leads to decreased stimulation of the adrenal glands and thus decreased cortisol production. This statement is True.
• Reason (R): Secondary adrenal insufficiency is indeed characterized by pituitary failure, leading to insufficient ACTH secretion. This statement is True.
• Explanation: The reduced ACTH secretion due to pituitary failure directly causes the adrenal glands to produce less cortisol, explaining the decreased plasma cortisol levels. The reason correctly explains the assertion.

Therefore, the answer is A.

2. Assertion (A): Aldosterone secretion increases in response to high blood volume.
Reason (R): Activation of the renin-angiotensin system stimulates aldosterone release from the adrenal cortex.

• Assertion (A): Aldosterone secretion decreases in response to high blood volume to promote sodium and water excretion and reduce blood volume. It increases in response to low blood volume. This statement is False.
• Reason (R): Activation of the renin-angiotensin-aldosterone system (RAAS) is a primary stimulus for aldosterone release from the adrenal cortex. This statement is True.

Therefore, the answer is D.

3. Assertion (A): Patients with primary hyperaldosteronism typically present with hypokalemia.
Reason (R): Aldosterone promotes potassium and sodium retention in the distal tubules.

• Assertion (A): Primary hyperaldosteronism (Conn's syndrome) involves excessive aldosterone, which leads to increased potassium excretion by the kidneys, resulting in hypokalemia. This statement is True.
• Reason (R): Aldosterone promotes sodium retention and potassium excretion (not retention) in the distal tubules and collecting ducts. This statement is False.

Therefore, the answer is C.

4. Assertion (A): Congenital adrenal hyperplasia due to $11\beta$-hydroxylase deficiency causes hypertension.
Reason (R): Deficiency leads to accumulation of deoxycorticosterone, which has mineralocorticoid activity.

• Assertion (A): $11\beta$-hydroxylase deficiency leads to the accumulation of precursors like deoxycorticosterone (DOC), which has mineralocorticoid activity, causing sodium and water retention and thus hypertension. This statement is True.
• Reason (R): The deficiency of $11\beta$-hydroxylase blocks the conversion of DOC, leading to its accumulation. DOC is a potent mineralocorticoid. This statement is True.
• Explanation: The accumulation of DOC, with its mineralocorticoid activity, directly causes the sodium and water retention that results in hypertension. The reason correctly explains the assertion.

Therefore, the answer is A.

5. Assertion (A): Dexamethasone suppression test is used to differentiate between adrenal and ectopic causes of Cushing's syndrome.
Reason (R): Dexamethasone is a synthetic glucocorticoid that suppresses ACTH secretion from the pituitary.

• Assertion (A): The high-dose dexamethasone suppression test is used to differentiate pituitary-dependent Cushing's disease from adrenal tumors and ectopic ACTH production. This statement is True.
• Reason (R): Dexamethasone is a potent synthetic glucocorticoid that exerts negative feedback on the pituitary, suppressing ACTH secretion. This statement is True.
• Explanation: The mechanism by which dexamethasone suppresses ACTH from the pituitary is fundamental to how the test differentiates between the various causes of Cushing's syndrome. The reason correctly explains the assertion.

Therefore, the answer is A.