Who should be sued when a patient is missed diagnosed by a MD working under a HMO?

The patient reported unexplained pain and numbness to upper back and arm. PMD diagnosed as a pulled muscle despite the patient informing him of no recent injury/trauma/heavy lifting and giving both her own and familial medical history. MD's only ordered treatment was a referral to physical therapy and Motrin, no diagnostic testing was done despite patient's request. Patient returns to the MD with increased pain and due to urging of a family member and MD again refuses to order any diagnostic test and continues with same diagnoses. Physical therapist asks the patient for diagnostic results and agreed that it should be done prior to ordered therapy. Patient explains continued pain to another doctor at the same facility during another visit. This doctor observes swelling in the reported area but still follows PMD's diagnoses and does not order any test. Pt lives with increased pain for months before administration gets involved. Finally test were done,results were Advanced Lung Cancer