Subacute Cutaneous Lupus Erythematosus (SCLE): This diagnosis is considered the most likely due to the patient's symptoms, including a photosensitive rash on the face, hands, and chest, which are typical locations for SCLE. The rash's description as erythematosus, round, scaling papules also aligns with SCLE. Additionally, the patient's joint pain and generalized malaise are common systemic symptoms associated with lupus.

Systemic Lupus Erythematosus (SLE): While SCLE is a subtype of lupus that primarily affects the skin, SLE is a more systemic form of the disease. The patient's symptoms of joint pain, low-grade fever, and generalized malaise, along with the characteristic butterfly-shaped rash on the face, could suggest SLE, especially if other systemic manifestations are present.
Erythema Multiforme: This condition is characterized by target-like lesions and can be triggered by viral infections or other factors. The patient's recent exposure to a large crowd and potential viral infections, along with the description of the rash, makes erythema multiforme a possible diagnosis.
Dermatomyositis: This is an inflammatory disease characterized by skin rash and muscle weakness. The patient's rash and joint pain could be indicative of dermatomyositis, although muscle weakness is not mentioned.

Lyme Disease: Although less likely given the patient's recent history and symptoms, Lyme disease can cause a rash (erythema migrans) and systemic symptoms like fever and joint pain. It's crucial to consider this diagnosis due to its potential for serious complications if left untreated.
Rocky Mountain Spotted Fever (RMSF): This tick-borne illness can present with fever, headache, and a characteristic rash. Given the patient's outdoor activities, RMSF is a critical diagnosis not to miss due to its potential severity and the need for prompt treatment.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: These are severe skin conditions usually triggered by medications or infections. They can present with skin lesions, fever, and systemic symptoms. Although the patient's symptoms do not fully align with these conditions, their potential severity warrants consideration.

Sjögren's Syndrome: An autoimmune disorder that can cause skin rashes, joint pain, and systemic symptoms. It's less common and typically associated with dry eyes and mouth, which are not mentioned in the patient's presentation.
Relapsing Polychondritis: A rare autoimmune disorder that can cause skin lesions, joint pain, and other systemic symptoms. The patient's symptoms could potentially align with this diagnosis, but it is much less common than other considerations.
Pemphigus: A group of autoimmune diseases characterized by blistering of the skin and mucous membranes. While the patient's rash does not primarily describe blisters, pemphigus is a rare condition that could be considered in a differential diagnosis for skin conditions with systemic symptoms.