Excretory product of birds and reptiles is

The most poisonous excretory product is _____

Of the following structures namely:

$\left(1\right)$Bowman's capsule

$\left(2\right)$ Proximal convoluted tubule

$\left(3\right)$ Henle's loop

$\left(4\right)$ Distal convoluted tubule

$\left(5\right)$ Collecting tubule

Which is present in the medulla of the kidney$?$