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: Medicine
Angelman syndrome is caused by: 
Select and explain. 
A. Maternal imprinting, B. Paternal imprinting, C. Both of the above, D. None of the above
A: Maternal imprinting
Answer: A: Maternal imprinting 
Explanation: Two microdeletion syndromes, Prader-Willi syndrome (PWS) and Angelman syndrome (AS), exhibit parent-of-origin, or "imprinting," effects. For many years, it has been known that cytogenetically detectable deletions of chromosome 15 occur in a propoion of patients with PWS, as well as in those with AS. New insight into the pathogenesis of these disorders has been provided by the recognition that parental origin of the deletion determines which phenotype ensues: if the deletion is paternal, the result is PWS, whereas if the deletion is maternal, the result is AS Ref: Harrison's principle of internal medicine 17th edition, chapter 64.