<prompt> Neostigmine is not able to cross blood brain barrier because of its:Primary structureSecondary structrureTertiary structureQuarternary structure  <response>: Ans. D Quarternary structureRef: KDT 8the p116Quarternary ammonium compounds are water soluble and thus cannot cross blood brain barrier. Neostigmine is a quarternary derivative whereas physostigmine is a tertiary amine. 