This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Glaucoma, Subject name: Ophthalmology
Tonography is used to determine 
Select and explain. 
A. The lOP at different times, B. The facility of aqueous outflow, C. The rate of formation of aqueous, D. None of the above
B: The facility of aqueous outflow
Answer: B: The facility of aqueous outflow 
Explanation: Tonography Tonography is a non-invasive technique for determining the facility of aqueous outflow (C-value). The C-value is expressed as aqueous outflow in microlitres per minute per millimetre of mercury. It is estimated by placing Schiotz tonometer on the eye for 4 minutes. For a graphic record the electronic Schiotz tonometer is used. C-value is calculated from special tonographic tables taking into consideration the initial IOP (P0) and the change in scale reading over the 4 minutes. Ref:- A K KHURANA; pg num:-481