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: Ophthalmology
The normal pH of tears is: March 2007 
Select and explain. 
A. 6, B. 6.5, C. 7, D. 7.5
D: 7.5
Answer: D: 7.5 
Explanation: Ans. D: 7.5 Human tear film/ precorneal film has three distinct layers, from the most outer surface: I. Lipid layer secreted by meibomian glands (or tarsal glands) - coats the aqueous layer; provides a hydrophobic barrier that evaporates and prevents tears spilling onto the cheek. These glands are found among the tarsal plates. Thus, the tear fluid deposits between the eye proper and oil barriers of the lids II. Aqueous layer- secreted by lacrimal gland - promotes spreading of the tear film; promotes the control of infectious agents; promotes osmotic regulation III. Mucous layer- secreted by conjunctival goblet cells - coats the cornea;provides a hydrophilic layer;allows for even distribution of the tear film; covers the cornea