Pressure-induced bruising: This is the most likely diagnosis, as the patient's symptoms are directly related to sitting on horizontal chairs, suggesting that pressure on the skin is causing the bruising.

Easy bruising syndrome: This condition is characterized by an increased tendency to bruise, often due to minor trauma or even spontaneously. The patient's history of bruising after sitting on chairs could be indicative of this syndrome.
Vitamin deficiencies: Deficiencies in vitamins such as vitamin C or vitamin K can lead to impaired collagen synthesis or blood clotting, respectively, making it easier for bruises to form.
Medication side effects: Certain medications, such as anticoagulants, antiplatelet agents, or steroids, can increase the risk of bruising.

Bleeding disorders: Conditions like hemophilia or von Willebrand disease can cause easy bruising and bleeding, and it is crucial to rule out these potentially life-threatening conditions.
Abuse or trauma: It is essential to consider the possibility of non-accidental injury, especially if the bruising pattern is suspicious or if there are other signs of abuse.
Malignancy: Certain types of cancer, such as leukemia or lymphoma, can cause easy bruising due to bone marrow suppression or platelet dysfunction.

Ehlers-Danlos syndrome: This genetic disorder affects collagen production and can lead to easy bruising, among other symptoms.
Pseudoxanthoma elasticum: A rare genetic disorder that affects the skin, eyes, and cardiovascular system, and can cause easy bruising due to fragile blood vessels.
Senile purpura: A condition characterized by easy bruising due to age-related changes in the skin and blood vessels.