Amyloidosis is generally described as the build up of abnormal protein called amyloid in the patient’s tissues and organs. One of the area affected can be a patient’s nail bed and appears as darken linear lines known as “longitudinal melanonychia.”
Dermatologic causes of melanocytic activation include psoriasis, lichen planus, amyloidosis, systemic lupus erythematosus, onychomycosis, and nonmelanocytic tumors.
Systemic causes of melanocytic activation include endocrine disorders such as Addison’s disease and Cushing’s syndrome, hyperthyroidism, and acromegaly. Other possible culprits include vitamin B12 deficiency, malnutrition, alcaptonuria, porphyria, graft-versus-host disease, AIDS, Laugier-Hunziker syndrome, and Peutz-Jeghers syndrome.
Iatrogenic causes of melanocytic activation include radiation and a host of commonly used medications, including antimalarials, minocycline, sulfonamides, ketoconazole, zidovudine, and chemotherapy agents.
Certain racial groups are susceptible including African Americans, Chinese, Japanese, and Native Americans. Melanocytes are cells that gives colour to the human hair, skin and nail. The pigment that is secreted by melanocytes is what’s more popularly known as melanin. The appearance of light to dark brown stripes that are parallel and regular in color, thickness, and spacing (refer to photos) can highlight a present condition as mentioned.
For more details please refer to article written by,
Dr Huang of Harvard Medical School,(https://www.verywellhealth.com/what-is-longitudinal-melanonychia-1069479)