What is the test for multiple causes in a personal injury action?

In discussing the problem of multiple causes one court stated, "The law is well settled that in a personal injury action causation must be proven [31 Cal.App.4th 1316] within a reasonable medical probability based upon competent expert testimony. Mere possibility alone is insufficient to establish a prima facie case. [Citations.] That there is a distinction between a reasonable medical 'probability' and a medical 'possibility' needs little discussion. There can be many possible 'causes,' indeed, an infinite number of circumstances which can produce an injury or disease. A possible cause only becomes 'probable' when, in the absence of other reasonable causal explanations, it becomes more likely than not that the injury was a result of its action. This is the outer limit of inference upon which an issue may be submitted to the jury. [Citation.]" (Jones v. Ortho Pharmaceutical Corp. (1985) 163 Cal.App.3d 396, 402-403, 209 Cal.Rptr. 456.)