This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Microbiology Pa 1 (History, Microscopy, Stains and Structure and Physiology of Bacteria), Subject name: Microbiology
Which of the following is a constituent of MacConkey agar? 
Select and explain. 
A. Lactose, B. Casein, C. Neutral Red, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Composition of MacConkey Agar : 1. Enzymatic digest of Gelatin, Casein and Animal tissue: provides nitrogen, vitamins, minerals and amino acids essential for growth. 2. Lactose: fermentable carbohydrate providing carbon and energy. 3. Bile Salts: selective agents and inhibit Gram positive organisms. 4. Crystal Violet:Gram positive bacteria are generally inhibited by crystal violet. 5. Sodium Chloride: supplies essential electrolytes for transpo and osmotic balance. 6. Neutral Red: pH indicator. Which is red in color at pH's below 6.8. When lactose is fermented, the pH of the medium decreases, changing the color of neutral red to pink 7. Agar : Solidifying agent