Contact Seth Director here on AVVO.
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You can, and I have handled several of these cases, but the burden is on you to show that at the time you were separated you should have been medically boarded. It is a significant burden to overcome, but it can be done. It would be worth your while to have a military law attorney review all of your discharge documents and give you an opinion on the viability of such a case.
Agree with the above, Mr. Cassara should be able to provide guidance as to whether it is worthwhile to pursue such an action. My suggestion is you make a request for the entirety of your VA Claims File with your VA Regional Office now so that those documents are in your possession if you choose to go this route. The VA had to make a determination that there was credible stressor "markers" in order to establish service-connection for MTS PTSD and you'll definitely need those documents going forward. I can provide a sample FOIA/Privacy Act request if you email me at firstname.lastname@example.org