What is the most likely diagnosis in a patient with rash, proximal muscle weakness, and fatigue?

The presentation of a rash, proximal muscle weakness, and fatigue is highly characteristic of dermatomyositis. This condition is an inflammatory myopathy associated with distinctive skin changes, such as heliotrope rash (a purplish rash on the eyelids) or Gottron's papules (erythematous papules over the joints). The proximal muscle weakness typically affects the muscles closest to the trunk, which can lead to difficulties with activities such as climbing stairs or lifting objects.

In dermatomyositis, the combination of muscle weakness and specific skin findings helps in distinguishing it from other conditions. For example, while systemic lupus erythematosus can also present with a rash and fatigue, it usually encompasses a broader range of systemic symptoms and does not primarily present with significant muscle weakness. Guillain-Barré syndrome primarily presents with ascendant muscle weakness and reduced reflexes due to nerve demyelination, often following an infectious illness, and lacks the specific skin manifestations seen in dermatomyositis. Psoriasis typically involves well-defined scaly plaques and does not cause the kind of muscle weakness associated with dermatomyositis.

Thus, the combination of rash, proximal muscle weakness, and fatigue consistently points towards dermatomyositis as the most likely