Am I entitled to a second opinion for pre-employment medical screening?

No, unfortunately you are not entitled to a second opinion. An employer is able to set its own terms or conditions for employment, including that a prospective employee take--and pass--a pre-employment medical screening. The employer can also decide who will do the screening and is not obligated to accept a second opinion.

However, what is potentially in your favor is that employers may *not* discrminate against people with disabilities (defined fairly broadly), so long as the person can do the job (either as is, or with some reasonable accomodation--e.g. someone with vision impairment might be able to do the job perfectly well with a computer screen that magnifies its images). The physical can therefore only ding or downcheck you for factors concerned with or related to job performance. For example: say the job is one that requires extensive travel (e.g. driving all over to visit other locations or clients). If the physical reveals that you can't sit for extended times, and therefore can't do the job since you can't drive for hours, that would be a valid reason to not give you the job. If however the physical revealed diabetes that's well under control, that's a nonissue for the job and you should probably not be denied it.

If you think you were denied the job for improper reasons, you should consult with an employment attorney who can evaluate the grounds for the denial and see if you perhaps have a legal claim.