This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
A 26 year old female patient complains of discoloured teeth. Intra-oral examination reveals generalized intrinsic tetracycline stains. What is the grade of staining in this patient? 
Select and explain. 
A. Grade I, B. Grade II, C. Grade III, D. Grade IV
D: Grade IV
Answer: D: Grade IV 
Explanation: None