A large bruise appeared after being bitten by a mosquito.

A large bruise after a mosquito bite may be caused by conditions such as allergic purpura, hyperpigmentation, or subcutaneous hemorrhage, and treatment should be based on the specific underlying cause. Detailed analysis is as follows:

1. Allergic Purpura

Patients present with skin purpura, symmetrically distributed, often accompanied by skin edema and urticaria. The purpuric lesions vary in size. Preventing infection is important; antihistamines should be used as directed by a physician, such as ipratropium bromide hydrochloride, or chlorpheniramine maleate, vitamin C, and other medications. Drugs that improve vascular permeability, such as troxerutin, may also be used.

2. Subcutaneous Hemorrhage

Local skin may be irritated by venom, and scratching can lead to localized swelling, further increasing the fragility of blood vessels in the subcutaneous tissue. Mild bruising usually resolves spontaneously and does not require special treatment. In more severe cases, cold compresses may be applied to help reduce subcutaneous bleeding.

3. Hyperpigmentation

Localized hyperpigmentation may occur after an insect bite, appearing as a discolored patch. This is typically temporary and may be accompanied by mild itching or other symptoms. If pigmentation persists for a long time or if rapid resolution is desired, medications such as hydroquinone cream or salicylic acid ointment may be used under a doctor's guidance.

In addition, similar symptoms may also result from insect bite dermatitis, thrombocytopenia, or secondary pressure-induced purpura. It is recommended that patients visit a hospital to determine the exact cause and receive timely treatment.