Intertrigo: This is a common condition characterized by a red, plaque-like rash in the skin folds, often due to friction, moisture, and fungal or bacterial overgrowth. The flexor surfaces of the arms are a typical location, making intertrigo a strong candidate for this presentation.

Contact Dermatitis: An allergic or irritant reaction to substances coming into contact with the skin could cause a red, plaque-like rash. Given the location on the flexor surfaces, exposure to allergens or irritants in this area is plausible.
Seborrheic Dermatitis: Although more commonly associated with the scalp, face, and torso, seborrheic dermatitis can occasionally present in flexural areas, causing a red, plaque-like rash.
Psoriasis (Inverse): Inverse psoriasis presents as smooth, red patches in skin folds, which could fit the description provided. It's less common than plaque psoriasis but should be considered, especially if other signs of psoriasis are present.

Erythema Multiforme: A skin condition characterized by target lesions, but it can also present with a more generalized rash. Although less likely, missing this diagnosis could be significant due to its potential association with serious underlying conditions.
Staphylococcal Scalded Skin Syndrome (SSSS): Caused by staphylococcal toxins, SSSS can present with a red, tender rash that can progress to blistering and peeling. It's more common in children but can occur in adults, especially those with compromised immune systems.

Granuloma Gluteale Infantum (or Adultum): Although typically found in the diaper area of infants, a similar condition can occur in adults, particularly in skin folds. It presents as reddish-brown plaques or nodules.
Hailey-Hailey Disease (Familial Benign Pemphigus): An inherited blistering skin condition that can present with red, erosive plaques in skin folds, including the flexor surfaces of the arms. Its rarity and genetic basis make it a less likely but interesting consideration.