What makes a tax "efficient"? Describe the efficiency properties of a lump sum tax

Under what conditions might a regular unit tax approximate a lump-sum tax? List one trade-off facing policymakers in attempting to minimized the excess burden of taxation.

A tax is considered efficient if it minimizes the welfare deadweight loss of taxation. A lump sum tax is considered to be the most efficient tax because it completely eliminates the deadweight loss of taxation because there is no way an individual could change her behavior to avoid the tax. Other taxes could approximate a lump sum tax if they were placed in a market with a perfectly inelastic supply or demand curve. One way to minimize the deadweight loss of taxation is to tax inelastic goods. However, taxing inelastic goods such as health care expenditures often can conflict with equity goals.