This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nasolacrimal Disorders, Subject name: Ophthalmology
Phenol red test/ Methyl red thread test for dry eye, true is 
Select and explain. 
A. Need instillation of anesthesia before doing the test, B. Need a ph meter to interpret, C. Color changes if there is mucin deficiency, D. Color changes from white to red on contact with tear film and is used to measure the dryness status of eye
D: Color changes from white to red on contact with tear film and is used to measure the dryness status of eye
Answer: D: Color changes from white to red on contact with tear film and is used to measure the dryness status of eye 
Explanation: Phenol Red Thread test is the test to detect dry eye severity. When in contact with tear film it changes its colour from white to yellow to red. Its main advantage over Schirmertest is: a)It is Less time consuming as compared to schirmer test(15 sec) b)Anaesthesia of cornea is not required c)On the contact of tear strip with the tear it turns red d)Can be done in children easily e)Reflex tearing is minimal f)Differentiates between aqueous deficient & non aqueous deficient eyes