**Core Concept**
The kidney plays a crucial role in maintaining potassium homeostasis, and the distal nephron is the primary site of potassium secretion. The distal nephron includes the collecting ducts and the cortical collecting ducts, where potassium ions are secreted into the lumen through the action of specific transport mechanisms.

**Why the Correct Answer is Right**
The correct answer choice is the portion of the nephron where the maximum amount of potassium ions are secreted. The cortical collecting ducts have a high density of principal cells, which express the sodium-potassium pump (Na+/K+-ATPase) and the potassium channel ROMK (renal outer medullary potassium channel). The Na+/K+-ATPase helps to pump sodium ions out of the cell and potassium ions into the cell, while ROMK allows potassium ions to leave the cell and enter the collecting duct lumen. This coordinated action enables the cortical collecting ducts to secrete a significant amount of potassium ions.

**Why Each Wrong Option is Incorrect**
**Option A:** The proximal convoluted tubule is primarily involved in the reabsorption of potassium ions, not secretion.
**Option B:** The loop of Henle is mainly involved in the reabsorption of sodium and chloride ions and the secretion of potassium ions, but not to the extent seen in the cortical collecting ducts.
**Option C:** The medullary collecting ducts are involved in the concentration of urine and the secretion of potassium ions, but the maximum secretion of potassium ions occurs in the cortical collecting ducts.

**Clinical Pearl / High-Yield Fact**
The kidneys play a critical role in maintaining potassium balance, and disturbances in potassium secretion can lead to hyperkalemia or hypokalemia. The use of certain medications, such as loop diuretics and potassium-sparing diuretics, can alter potassium secretion and lead to changes in serum potassium levels.

**Correct Answer: C. Cortical collecting ducts**