**Core Concept**
The principal site of acidification of urine is a critical aspect of renal physiology, involving the regulation of hydrogen ion secretion to maintain acid-base balance in the body. This process is essential for preventing metabolic acidosis and ensuring proper bone mineralization.

**Why the Correct Answer is Right**
The collecting duct is the principal site of acidification of urine due to the presence of alpha-intercalated cells, which secrete hydrogen ions into the lumen. The alpha-intercalated cells express the H+/K+ ATPase enzyme, which pumps hydrogen ions into the tubular lumen, thereby increasing the pH of the urine. This process is regulated by the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system.

**Why Each Wrong Option is Incorrect**
**Option A:** The distal convoluted tubule is involved in the reabsorption of calcium and magnesium ions, but it is not the primary site of acidification of urine.
**Option B:** The proximal convoluted tubule is responsible for the reabsorption of glucose, amino acids, and other nutrients, but it does not play a significant role in acidification of urine.
**Option C:** The Loop of Henle is involved in the concentration and dilution of the urine, but it is not the primary site of acidification of urine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the collecting duct's ability to acidify urine is influenced by the presence of aldosterone, which stimulates the expression of H+/K+ ATPase in alpha-intercalated cells. This is a critical aspect of managing patients with acid-base disorders.

**✓ Correct Answer: D. Collecting duct. Collecting duct is the principal site of acidification of urine.**