**Core Concept**
Primary aldosteronism is a condition characterized by the excessive production of aldosterone, a hormone that plays a crucial role in regulating sodium and potassium levels in the body. The primary cause of primary aldosteronism is usually an aldosterone-producing adenoma (APA) or bilateral adrenal hyperplasia, leading to an overactive renin-angiotensin-aldosterone system (RAAS).

**Why the Correct Answer is Right**
The correct answer, **C. Increased Na+**, is due to the direct effect of aldosterone on the kidneys. Aldosterone stimulates the distal convoluted tubules and collecting ducts of the nephrons to increase sodium reabsorption, leading to an increase in sodium levels in the blood. This, in turn, results in an increase in water reabsorption, causing hypertension and hypokalemia (low potassium levels). The increased sodium levels also lead to an expansion of the extracellular fluid volume, which contributes to the development of hypertension.

**Why Each Wrong Option is Incorrect**
**Option A:** Pedal oedema is typically associated with conditions that lead to fluid overload, such as heart failure or nephrotic syndrome. In primary aldosteronism, the increased sodium levels lead to volume expansion, but it is usually not severe enough to cause pedal edema.

**Option B:** Increased renin levels are actually a compensatory response to the excessive aldosterone production. In primary aldosteronism, the renin-angiotensin-aldosterone system (RAAS) is overactive, leading to an increase in renin levels, which in turn stimulates the production of angiotensin II, leading to further aldosterone production.

**Clinical Pearl / High-Yield Fact**
Primary aldosteronism is often referred to as "Conn's syndrome" after the physician who first described it. It is a common cause of secondary hypertension and can be diagnosed using the aldosterone-to-renin ratio (ARR) test.

**Correct Answer Line**
✓ Correct Answer: C. Increased Na+