You know I like to show how a native brain works, so, here some questions and answers:
¿Qué
es la capital de Perú?
Una ciudad
¿Qué
es tu razón
para esto?
Una razón
¿Qué
es la respuesta?
Una respuesta
Una frase
Remember "ser" is the substantive verb in Spanish, so it makes the substance of things. Ask "qué" and you'll get the substance. Ask "cuál" and you'll got one spotted among many of similar substance. When you ask "¿qué/cuál dedo? nothing 'substantive' is going on. Notice that no article is needed, on the contrary (¿qué el dedo?
¿cuál el dedo?
), but when a pronoun is involved we use "cuál" (¿cuál de ellos? / ¿cuál de tus dedos?), so what role plays "qué" in such a sentence. Well, I would say it just calls for attention or more information: give me any information that let me identify that thing among the blurry scene, even substantive info.