Hypothyroidism: This condition is characterized by dry skin, hair loss, and brittle nails, which aligns with the patient's symptoms. Smoking is also a risk factor for thyroid diseases, making hypothyroidism a plausible diagnosis.

Nutritional Deficiencies: Deficiencies in vitamins and minerals such as vitamin B, iron, or zinc can cause dry skin, hair loss, and brittle nails. Smoking can lead to poor dietary habits and decreased absorption of essential nutrients.
Smoking-Related Skin Changes: Smoking is known to cause premature aging of the skin, including dryness, wrinkles, and hair loss, due to its effects on blood flow and collagen production.
Atopic Dermatitis: This chronic skin condition can cause dry, itchy skin, and may also affect the hair and nails. While not directly related to smoking, atopic dermatitis is a common condition that could be exacerbated by smoking.

Sjögren's Syndrome: This autoimmune disorder can cause dry skin, hair loss, and brittle nails, as well as other systemic symptoms. While less common, Sjögren's syndrome can be severe and requires prompt diagnosis and treatment.
Cancer (e.g., Lung, Thyroid): Smoking is a significant risk factor for various types of cancer, including lung and thyroid cancer. While the patient's symptoms may not directly suggest cancer, it is essential to consider this possibility due to the patient's smoking history.

Pellagra: This rare condition is caused by a deficiency in niacin (vitamin B3) and can cause skin, hair, and nail changes, as well as other systemic symptoms. While unlikely, pellagra is a possibility, especially if the patient has a poor diet.
Acrodermatitis Enteropathica: This rare genetic disorder affects zinc absorption and can cause skin, hair, and nail changes, as well as other symptoms. While rare, this condition should be considered if other diagnoses are ruled out.