In a patient presenting with diffuse pain in the right arm and sensory loss in the right thumb, what is the most likely diagnosis?

The presentation of diffuse pain in the right arm along with sensory loss in the right thumb strongly suggests C6 radiculopathy. This specific condition occurs when there is compression or irritation of the C6 nerve root, which typically leads to pain and sensory changes in areas served by this nerve, including the radial aspect of the arm and the thumb.

In C6 radiculopathy, patients often experience pain that radiates from the neck or shoulder down the arm, potentially accompanied by weakness and sensory deficits in the thumb and index finger. The description of sensory loss in the right thumb aligns with the nerve root's dermatome responsible for sensation in this area.

Other options may cause arm pain or sensory loss but do not match the presentation as closely as C6 radiculopathy does. For example, carpal tunnel syndrome generally results in symptoms limited to the hand and particularly affects the median nerve distribution, not the entire arm. Pancoast tumor and thoracic outlet syndrome have more complex presentations and often involve additional signs, such as systemic features or involvement of different neurovascular structures. Therefore, the characteristics of the patient's symptoms correspond directly with C6 radiculopathy, making it the most likely diagnosis.