Serum marker of rickets is
Acid phosphates
Alkaline phosphatase
Decreased serum calcium
Urinary posphates
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.