Melasma: This condition is characterized by flat, dark, irregularly shaped macules, typically on sun-exposed areas such as the face, but can also appear on the arms. It's more common in women, especially during pregnancy or with hormonal changes, and in individuals with darker skin types.

Post-Inflammatory Hyperpigmentation (PIH): This occurs after inflammation or injury to the skin, leading to darkened areas that can be irregular in shape. It's common in areas prone to trauma or inflammation.
Solar Lentigines: Also known as sun spots or liver spots, these are flat, dark spots that appear due to sun exposure. They can appear on any sun-exposed area, including the arms.
Café-au-Lait Macules: These are benign, flat, light to dark brown spots that can appear anywhere on the body, including the arms. They are more noticeable in individuals with lighter skin.

Malignant Melanoma: Although less likely, it's crucial to consider melanoma, especially if the macule is changing in size, shape, or color, or if it exhibits the ABCDE signs (Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolving).
Drug-Induced Pigmentation: Certain medications can cause skin pigmentation changes, including flat, dark macules. This is particularly important if the patient has recently started new medications.

Erythema Dyschromicum Perstans: A rare condition characterized by the gradual appearance of ash-colored macules, typically on the face, neck, and arms. It's more common in Latin American women.
Acquired Bilateral Nevus of Ota-like Macules (Hori's Nevus): A rare condition presenting with bluish-gray macules, typically on the face but can also appear on the arms. It's more common in Asian women.
Dowling-Degos Disease: A rare genetic disorder that can cause reticular pigmentation of the skin, particularly in the flexures, but can also present with flat, dark macules on other areas, including the arms.