Mandy Moore says her toddler has a rare skin condition called Gianotti Crosti syndrome. What is that?

Actress Mandy Moore has revealed that her 2-year-old son August, nicknamed Gus, has been diagnosed with a rare skin condition called Gianotti Crosti syndrome.

Moore took to social media on Friday to describe a “crazy rash” Gus woke up with last week.

“We thought maybe an eczema flare? Poison Oak? Allergy,” she wrote on Instagram Stories. “We tried to deduce what it could be and did anything to help him find relief from the itch.”

The “This is Us” star said Gus was taken to urgent care, and after consultations with a pediatrician, a dermatologist and then a pediatric dermatologist, Gus was eventually diagnosed with GCS.

Moore, who shares Gus with her husband, musician Taylor Goldsmith from the band Dawes, said the experience is a reminder that parenting can be tough.

“Sometimes you feel so helpless (and yes I’m ever so grateful it’s only an itchy skin condition),” she wrote. “Kids are resilient and as long as he’s smiling through it, we are a-okay.”

What is Gianotti Crosti syndrome?

GCS is a rare childhood skin condition characterized by “a papular rash with blisters on the skin of the legs, buttocks, and arms,” according to the National Institutes of Health. “Papular” refers to raised bumps on the skin.

Those areas are exactly where Moore said Gus was experiencing symptoms.

“It’s all over his legs and feet (ouch) and the backs of his arms, but nowhere else,” she said, sharing a photo of the boy’s inflamed, rashy legs. 

Typically the skin lesions last at least 10 days, but they can last for several weeks, the NIH says on its website. 

The lesions are usually associated with an underlying infection, often a virus, that can cause other symptoms like a low-grade fever, sore throat or upper respiratory infection symptoms.

“GCS is thought to be a hypersensitive response to the underlying infection,” the NIH explains. “While in many countries the underlying cause is hepatitis B, this is rarely the cause in North America.”

The condition most often affects children between 9 months and 9 years of age, according to the National Organization for Rare Disorders.

And while the lesions usually go away on their own, sometimes treatments are given to help with symptoms.  

“In some cases, the use of topical ointments or certain medications by mouth may be recommended to help alleviate mild to potentially severe itching,” the organization’s website  notes.

Moore shared a similar description of the treatment plan she’s following for Gus.

“There’s nothing to do but a steroid cream and Benadryl at night…Anyone else ever experience this??” she wrote.

–Sara Moniusko contributed reporting.