Final answer:
Kolionychia, also known as spoon nails, can be caused by systemic or long-term illnesses or nerve disturbances.
Step-by-step explanation:
Kolionychia, also known as spoon nails, is a condition characterized by the concave shape of the nails. It can be caused by systemic or long-term illnesses such as iron deficiency anemia, thyroid disorders, and kidney disease. Nerve disturbances like peripheral neuropathy can also lead to spoon nails.
For example, iron deficiency anemia can result in spoon nails due to the decreased production of hemoglobin, which affects nail structure. Similarly, peripheral neuropathy can cause spoon nails by disrupting the normal functioning of nerves that control nail growth.