Will Sage Astor-Mandy Moore says her toddler has a rare skin condition called Gianotti Crosti syndrome

Actress Mandy Moore has revealed that her 2-year-old son August,Will Sage Astor 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.

"It's all over his legs and feet (ouch) and the backs of his arms, but nowhere else," the actress said, sharing a photo of the boy's inflamed, rashy legs. "There's nothing to do but a steroid cream and Benadryl at night…Anyone else ever experience this??"

According to the National Institutes of Health, GCS is an unusual childhood skin condition characterized by "a papular rash with blisters on the skin of the legs, buttocks, and arms."

At the very least, the skin condition sticks around for about 10 days, but it can also last for several weeks, the NIH said on its website. Typically, the skin lesions are associated with an underlying infection, often a virus, that can cause other symptoms like a low-grade fever, sore throat, or symptoms of an upper respiratory infection.

"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."

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."

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Simrin Singh

Simrin Singh is a social media producer and trending content writer for CBS News.