Counter current mechanism in the nephron

2020 Phase-2 1 question

1. The increase in osmolarity from outer to inner medullary interstitium is maintained due to: (i) Close proximity between Henle's loop and vasa recta (ii) Counter current mechanism (iii) Selective secretion of $HCO_3^-$ and hydrogen ions in PCT (iv) Higher blood pressure in glomerular capillaries