But a pattern soon emerges. Ruthie, her building’s super, worries about the tension Mary Jane is absorbing in her body. (“That’s how my sister got cancer,” she offers.) Sherry, Alex’s primary home care nurse, makes more of the boy’s temperature spikes than Mary Jane dares. Amelia, Sherry’s niece, is startled by Mary Jane’s exuberant warmth. And Brianne, the mother of a child with a similar condition, almost collapses under the weight of a can-do pep talk.

These women, brought to beautifully detailed life by Brenda Wehle, Liza Colón-Zayas, Danaya Esperanza and Susan Pourfar, act as a four-part treble chorus, reflecting Mary Jane’s disowned emotions. That they range in age from 21 to 70-ish suggests that Ms. Herzog is also exploring a larger issue, one that for Mary Jane predated Alex’s birth and, we begin to dread, will outlast him.

That larger issue comes into focus halfway through the play, when in a terrifying and beautifully staged moment, the set, by Laura Jellinek — the reigning Off Broadway master of Transformers-style reconfiguration — turns itself inside out just as the drama does. Now we are in a hospital, where Mary Jane can no longer domesticate her fears. The four supporting actors recur in new and symmetrical guises as her medical and spiritual advisers.

Ms. Herzog has often built her plays, from “4000 Miles,” to “After the Revolution” to “Belleville,” on the fault lines of belief: the lies, secrets and ideologies we all straddle to get by. Here, she goes further, approaching the most difficult matters of faith. In encounters with a sharp-tongued Hasidic mother of seven (Ms. Pourfar again) and a novice hospital chaplain (Ms. Wehle), Mary Jane begins to understand that the life-or-death questions her child’s illness has raised were always there anyway. Being a mother (the medical staff relentlessly refers to her not by name but as “Mom”) does not protect her, or anyone, from them.

This is why Chaya, the Hasidic mother, tells Mary Jane it is a relief to be in the hospital with her sick daughter: Everything else “wasn’t real.” And why the chaplain, a recent convert, admits that her checkered religious history isn’t the only thing that makes her life seem arbitrary. In fact, she says, “It’s pretty far down on that list.”

These two scenes feel brutally honest in part because they are brutally inconclusive. The play, under Ms. Kauffman’s ideally detailed direction, is not out to answer any questions. In that sense “Mary Jane,” with its ordinary name, is a character study, an Everywoman story, despite its origin in specific personal experience. Ms. Herzog and her husband, the director Sam Gold, have a child who was born in 2012 with a muscle disease called nemaline myopathy.