Learn Spanish (Spain): La timidez del niño es un problema, lo cual molesta a sus padres. - The boy's shyness is a problem, which bothers his parents.

The word timidez (shyness) is an abstract noun derived from the adjective tímido (shy). The suffix -ez is frequently added to adjectives to express a quality or state of being, much like '-ness' in English. Note that all abstract nouns ending in -ez are feminine, which is why it takes the article la.

Why does this sentence use lo cual instead of just que or el cual?

The phrase lo cual (which) is a neutral relative pronoun used to refer back to a complete idea or situation, not just a single noun. Here, it refers to the whole first clause: 'the boy's shyness is a problem'. If you used el cual, it would have to refer specifically to the masculine noun un problema. You could also use lo que here with the same meaning.

Why is it un problema instead of una problema when the word ends in -a?

Even at an intermediate level, this is easy to slip up on! Nouns ending in -ma that have Greek origins are masculine in Spanish. You will see this same pattern in other common words like el tema (the topic), el sistema (the system), and el idioma (the language).

Why do we need the preposition a before sus padres in this sentence?

This is the personal a. Whenever the object of a verb is a specific person or group of people (like his parents), Spanish requires this preposition before it. It does not translate to anything in English, but leaving it out sounds very unnatural in Spanish.