A patient with progressive proximal muscle weakness associated with a lung mass most likely has which of the following?

The presence of progressive proximal muscle weakness alongside a lung mass suggests a specific paraneoplastic syndrome associated with certain cancers, particularly small cell lung cancer (SCLC). Lambert-Eaton myasthenic syndrome (LEMS) is characterized by muscle weakness that primarily affects the proximal muscles, such as the muscles of the hips and shoulders, which aligns with the symptoms presented.

In LEMS, there is an autoimmune response that leads to impaired release of acetylcholine at the neuromuscular junction, resulting in muscle weakness. This condition is often associated with malignancies, especially small cell lung carcinoma. Patients with LEMS may also experience autonomic symptoms such as dry mouth and erectile dysfunction due to the involvement of the autonomic nervous system, which can differentiate it from other forms of myasthenia that are not linked to malignancy.

In contrast, myasthenia gravis is typically associated with weakness that improves with rest and is not commonly linked to lung cancers in the same way as LEMS. Multiple sclerosis presents differently, primarily affecting central nervous system function, leading to varied neurological symptoms rather than isolated muscle weakness with a lung mass. Guillain-Barré syndrome presents with an acute onset of muscle weakness and is associated with peripheral nerve dysfunction rather