Four-month-old Ainsley Cardente smiles, enjoys “helicopter” rides from her mom and protests when lunch is late. Perfectly ordinary, all of it. And her life is expected to remain that way thanks to a $2.1 million drug recently infused into her little body by doctors from Johns Hopkins Hospital.

Ainsley was born with a rare genetic disorder called spinal muscular atrophy, or SMA, that kills most babies by age 2. Rarely, there are less severe versions like the one that afflicts Ainsley that lead to a slower but still catastrophic decline.

“She’ll never be in a wheelchair; she’ll never feel like a sick person,” said Kate Cardente, Ainsley’s mother, a physician assistant from the Annapolis suburb of Arnold. “She’ll never remember it.”

But Ainsley almost didn’t get the medicine known as Zolgensma. On the market since May, it’s the nation’s most expensive prescription drug and the Cardentes’ insurer initially refused to cover the cost. So Cardente and her husband, David, fought to get it approved. A letter from Ainsley’s doctor at Johns Hopkins Hospital, a top SMA expert, helped win coverage.