Nail Sign observed in a 19 year old man

A 19-year-old man presented with pedal oedema and facial puffiness of 3 months’ duration. He denied history of orthopnoea, paroxysmal nocturnal dyspnoea, oliguria, haematuria, fever, sore throat, joint pains, rashes, exposure to heavy metals, or any drug abuse.

General physical examination showed periorbital swelling, pallor, nail changes, and pedal oedema. Systemic examination was unremarkable. Laboratory test results showed anaemia, hypoalbuminaemia, hyperlipidaemia, and normal renal and liver function tests. Urinalysis showed dysmorphic red cells and albuminuria (4+); 24-h urinary protein excretion was 5·10 g.

What is this nail sign called, and what could be the potential cause ?