Pityriasis versicolor (also known as tinea versicolor): This condition is characterized by circular, hypopigmented or hyperpigmented skin lesions, often found on the trunk, but can also appear on the limbs. The presence of lesions on the lower shin and the description provided aligns closely with pityriasis versicolor, especially considering its common occurrence in young adults.

Vitiligo: Characterized by well-defined, milky-white patches due to loss of melanocytes. The involvement near the nailbeds, as mentioned, is a common site for vitiligo, and the condition can also present with lesions on the limbs.
Tinea corporis (ringworm): A fungal infection that can cause circular, hypopigmented lesions. It's less likely given the specific locations mentioned but remains a possibility, especially if the patient has been exposed to fungi or has a history of similar infections.
Pityriasis alba: More commonly seen in children, this condition presents with ill-defined, hypopigmented patches, often on the face but can appear on the limbs. It's associated with mild scaling and is considered a form of eczema.

Leprosy (Hansen's disease): Although rare in many parts of the world, leprosy can cause hypopigmented skin lesions and is crucial to diagnose early due to its potential for nerve damage and disfigurement. The involvement of the skin and peripheral nerves, particularly if there's any sensory loss or weakness, would necessitate consideration of this diagnosis.
Sarcoidosis: A systemic disease that can manifest with skin lesions, including hypopigmented patches, among other symptoms like lung involvement and eye problems. It's essential to consider due to its systemic implications.

Idiopathic guttate hypomelanosis: A condition characterized by small, discrete, hypopigmented macules, typically on the extremities of older adults. While it matches some aspects of the description, the size and specific locations mentioned make it less likely.
Chemical or occupational leukoderma: Certain chemicals can cause hypopigmentation, similar to vitiligo. This would be considered if the patient has a history of exposure to specific industrial chemicals.
Tuberous sclerosis complex: A genetic disorder that can cause hypopigmented macules (ash leaf spots), among other systemic manifestations. The presence of other symptoms like seizures, intellectual disability, or specific skin findings like angiofibromas would raise suspicion for this condition.