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: Forensic Medicine
Dried semen stains on clothes are identified by the following method: 
Select and explain. 
A. Spectrometry, B. Ultraviolet rays, C. Infrared rays, D. Magnifying lens
B: Ultraviolet rays
Answer: B: Ultraviolet rays 
Explanation: Seminal stains when dry have a greyish-white or yellow-grey colour and show an irregular, map-like outline. When examined under filtered ultraviolet light, they show a fluorescence of a bluish-white colour, which is not specific as other albuminous materials such as nasal, leucorrhoeal discharges and detergents also fluoresce. Fluorescence depends on choline of semen. Ref: The essentials of forensic medicine and toxicology by Dr K S Narayan Reddy, 27th edition, Page 369.