Most likely causes

1. Minor repeated trauma (most common in toddlers). The big toe takes the most pressure from crawling, walking, kicking, tight socks or shoes, and rubbing on floors. Damage to the nail matrix causes white patches, transverse bands and brittle, splitting nails. Fitzpatrick's describes small white patches or transverse bands (leukonychia) as mainly seen in children, probably from repeated minor trauma. - Fitzpatrick's Dermatology, Chromonychia section
2. Beau lines (horizontal grooves). These are transverse furrows caused by a temporary pause in nail matrix function. The groove moves forward as the nail grows. Triggers include any febrile or systemic illness, hand-foot-and-mouth disease and other viral infections, a major injury, and drug reactions. - Andrews' Diseases of the Skin, Beau Lines
   If your son had a fever or illness about 2 to 3 months ago, this fits. A toenail grows out slowly, so a groove can appear long after the illness.
3. Fungal infection (white superficial onychomycosis). This causes a chalky, white, crumbly surface. It is uncommon at 1.5 years, but it is possible. It is more likely if a family member has nail fungus or athlete's foot. Fitzpatrick's describes this white fungal surface change as "pseudoleukonychia." A 2025 review of pediatric dermatophyte onychomycosis notes it is generally uncommon in children.
4. Less likely causes:
   • Nail psoriasis or eczema around the nail
   • Inherited nail disorders (these usually involve several nails)
   • Rarely, nutritional deficiency. Zinc or iron deficiency would also affect other nails and the child's general health.

What to look at

• Only the big toe or several nails? A single nail points to trauma or fungus. Many nails point to illness, Beau lines or an inherited cause.
• Does the groove sit at the same level on every affected nail? That suggests one past illness.
• Is the nail thickened, yellow, crumbly or lifting from the nail bed? That raises the chance of fungus.
• Any redness, swelling, pus or pain around the nail? That suggests infection.
• Other signs: Check for skin rashes, hair loss, poor weight gain or a pale child.

What you can do at home

• Keep the nails short and smooth, and file rough edges.
• Use soft, well-fitting shoes and socks.
• Keep the feet clean and dry.
• Do not pick or peel the nail.
• Do not apply any antifungal or home remedy without medical advice.
• Watch the nail for 2 to 3 months. A nail that is simply damaged grows out normally from the base.

When to see a pediatrician or dermatologist

• The new nail growing from the base still looks abnormal.
• The nail becomes thick, yellow or crumbly, or lifts off.
• There is redness, swelling, pus or pain at the nail fold.
• Several nails are involved, or there are other symptoms such as rash, poor growth or recurrent illness.
• The change doesn't improve in 2 to 3 months.