What is the burden of proving a disability under the Social Security Act?

A disability is defined by the Social Security Act as an "inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment." 42 U.S.C. 423(d)(1)(A). The claimant has the burden to show that she "suffers from a severe impairment that renders [her] disabled." Ferraris v. Heckler, 728 F.2d 582, 584 (2d Cir.1984). "[I]f the claimant suffers from an impairment listed in 20 C.F.R. Part 404, subpart P, Appendix 1, that ... ends the inquiry and the claimant is determined to be disabled." Id. Alternatively, "if the claimant suffers from a severe impairment but one not listed as presumptively disabling, [she] must show that [she] is unable to continue [her] past relevant vocational work." Id.