**Core Concept**
The principal site of acidification of urine is a critical process in maintaining acid-base balance in the body. It involves the reabsorption of bicarbonate ions in the proximal convoluted tubule and the subsequent excretion of hydrogen ions in the distal nephron, primarily in the collecting duct.

**Why the Correct Answer is Right**
The correct answer is the collecting duct, where the hydrogen ion secretion is mediated by the enzyme carbonic anhydrase and the sodium-hydrogen exchanger (NHE3). The collecting duct's acidification of urine is regulated by the sympathetic nervous system and aldosterone, which increases sodium reabsorption and hydrogen ion secretion. The collecting duct's ability to acidify urine is essential in maintaining acid-base balance, particularly in the presence of metabolic acidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** The proximal convoluted tubule does reabsorb bicarbonate ions, but it is not the primary site of acidification of urine. The proximal convoluted tubule's bicarbonate reabsorption is mediated by the sodium-bicarbonate cotransporter (NBC).
**Option B:** The loop of Henle does have a role in acid-base regulation, but it is not the primary site of acidification of urine. The loop of Henle's acidification is primarily mediated by the sodium-potassium-chloride cotransporter (NKCC2).
**Option C:** The distal convoluted tubule does have some ability to acidify urine, but it is not the primary site. The distal convoluted tubule's acidification is mediated by the sodium-hydrogen exchanger (NHE3) and the potassium-hydrogen exchanger (NHE1).

**Clinical Pearl / High-Yield Fact**
The ability of the collecting duct to acidify urine is impaired in patients with chronic kidney disease and those taking loop diuretics, such as furosemide. This can lead to a metabolic acidosis, which can be treated with bicarbonate supplementation.

**Correct Answer:** C.