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: Pharmacology
MDR gene acts by - 
Select and explain. 
A. Block drug activation, B. Block intracellular DNA synthesis, C. Causes efflux of drug, D. DNA repair inhibition
C: Causes efflux of drug
Answer: C: Causes efflux of drug 
Explanation: Ans. is 'c' i.e., Causes efflux of drug Multi drug resistance o Resistance develops to a variety of anticancer drugs. o It is associated with increased expression of a normal gene MDR1 gene. o MDR I gene encodes for P-glycoprotein Pcgp). P glycoprotein is a protective mechanism possessed by normal cells. o Pgp is an ATP dependent membrane efflux pump. o Its function is to expel a variety offoreign molecules including antitumour drugs from the cell. o Long lived cells such as the haemopoietic stem cells, Cells on excretory surfaces such as biliary hepatocytes, proximal renal tubules and intestinal cells and the cells of the blood-brain barrier, all have high expression of Pgp. o The MDR phenomenon illustrates how tumour cells adapt and enhance normal cell mechanism to deal with the effects of chemotherapy and how repeated cycles of chemotherapy select out a population of cells which have developed adaptive survival mechanism e.g. in myeloma where MDR proteins are rare at diagnosis but common at progression.