Which part of the nephron is primarily responsible for ultra-filtration, the initial stage of urine formation?

The correct answer is A.

Ultra-filtration is the initial step in the formation of urine, where blood is filtered to remove waste products and excess water. This process occurs in the renal corpuscle, which consists of two main parts:

• The glomerulus: a network of capillaries where blood is filtered.
• Bowman's capsule: a cup-shaped structure that surrounds the glomerulus and collects the filtered fluid (glomerular filtrate).

The high blood pressure within the glomerulus forces water and small solutes from the blood into Bowman's capsule, while larger molecules like proteins and blood cells are retained in the blood.

The other options are involved in subsequent stages of urine formation:

• Distal convoluted tubule and Proximal convoluted tubule are involved in selective reabsorption and secretion.
• Collecting duct is involved in the final concentration of urine.

Therefore, ultra-filtration takes place in Bowman's capsule.

The final answer is $\boxed{\text{A}}$.