Serum marker of rickets is
Amongst the given options, alkaline phosphatase is best because it is raised in all type of rickets.
The primary (Basic) investigations in a child of rickets are :-


Serum calcium
Serum phosphorus
Alkaline phosphatase


If the patient does not respond to calcium and Vit D therapy, secondary investigations are done:-


Vit D level
Serum PTH
Urinary calcium and phosphorus.
Acid phosphates
Alkaline phosphatase
Decreased serum calcium
Urinary posphates