。
She became a doctor.
Literal
She [topic-は] doctor [to-に] became.
A minimal frame for the pattern noun + に + なる ('become X'). The destination/result of the change is marked with に, even when the change is metaphorical (becoming a profession, a state, a member of a category). 医者になる is a frequent topic in family expectations, career advice, and biographical retellings — Japanese culture has long invested the medical profession with prestige, and parental hopes for 'medical-school child' (医者の子) are an enduring trope.