Your suggestion is correct, m not calling it wrong, rather I said that the use of 'a' isn't wrong here ..

And in many sentences, there can be 'of', and it could be showing possession as well, still 'a' would be used, not 'the'.

For example,

1. He is a good friend of mine.
Here this sentence means that I have many friends and he is one of them.

2. It is a leaf of a mulberry tree.
This sentence, like the first one, means that a mulberry tree has many leaves, and it is or may be one of them.

Similarly,
Childhood is a stage of a person's development means that a person's development has many stages and Childhood is one of them.

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"So my birth was the first of all my misfortunes". (Jean Jacques Rosseou)