Which of the situations below will enhance microtubule shrinkage? (a) addition of a drug that inhibits gtp exchange on free tubulin dimers (b) addition of a drug that inhibits hydrolysis of the gtp carried by tubulin dimers (c) addition of a drug that increases the affinity of tubulin molecules carrying gdp for other tubulin molecules (d) addition of a drug that blocks the ability of a tubulin dimer to bind to γ-tubulin

The correct answer is option a.

Explanation:

When the process of hydrolysis moves towards the tip of the microtubule, it starts brisk depolymerization and shrinkage. The addition of GTP-bound tubulin to the tip of the microtubule again offers a new cap and protects the microtubule from shrinking.  

The addition of a drug prevents GTP exchange on free tubulin dimers, that is, the drug, which prevents GTP exchange on free tubulin dimers will efficiently minimize the availability of the pool of GTP-bound tubulin dimers for addition to microtubule terminals. Thus, favoring microtubule disassembly or microtubule shrinkage.  

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