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
Which pa of the lens contains the oldest cells? 
Select and explain. 
A. Nucleus, B. Anterior surface of lens, C. Posterior surface of lens, D. Nucleo-coical junction
A: Nucleus
Answer: A: Nucleus 
Explanation: The lens is a biconvex avascular structure suspended by thin filamentous zonules attached to the ciliary process between the iris anteriorly and vitreous humor posteriorly. The nucleus which is the innermost pa of the lens contains the oldest cells. Newly laid fibers crowd and compact around previous fibers. The outermost fibers constituting the lens coex are the most recently formed fibers. In lens fibers aerobic metabolism is absent. The region between the epithelial monolayer and the nucleus is the coex which is relatively younger and apa from epithelium the most metabolically active region of the lens. Ref: Manual of Ocular Diagnosis and Therapy edited by Deborah Pavan-Langston page 152. Redox-Genome Interactions in Health and Disease edited by Jurgen Fuchs, page 339.