What is the test for a defendant's Fifth Amendment right to refuse to testify at trial?

Finally, Corral argues, incorrectly, that the prosecutors statement: "[a]nd given all the evidence that was discovered there, very direct evidence, the dope the guns, the scales, the cash, the lack of any credible explanation for that other than this man is in the drug business," was an impermissible comment on his failure to testify. A prosecutor may comment on the absence of exculpatory evidence without implicating a defendant's Fifth Amendment. United States v. Mares, 940 F.2d 455, 461 (9th Cir.1991).