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: Pathology
Organ which undergoes involution: March 2005 
Select and explain. 
A. Parathyroid gland, B. Thyroid gland, C. Pineal gland, D. Thymus
D: Thymus
Answer: D: Thymus 
Explanation: Ans. D: ThymusOne of the major characteristics of veebrate immunology is thymic involution, the shrinking of the thymus with age, resulting in changes in the architecture of the thymus and a decrease in tissue mass.This process has been conserved in almost all veebrates, from birds, teleosts, amphibians to reptiles, though the thymuses of a few species of sharks are known not to involute.Since the thymus is the organ responsible for T-lymphocyte (T cell) maturation, its regression has been linked to the reduction in immunosurveillance in the elderly.Though thymic involution has been linked to senescence, it is not induced by senescence as the organ stas involuting from a young age - as early as the first year of life in humans