Learn Italian: Se ci sono sintomi gravi, il medico la prescrive. - If there are serious symptoms, the doctor prescribes it.

la is a direct object pronoun meaning 'it'. It is feminine singular, telling us that 'it' refers to a feminine noun from the wider context, such as la medicina (the medicine) or la cura (the treatment). In Italian, these object pronouns are placed immediately before the conjugated verb.

What kind of 'if' clause is used in this sentence?

This is a first conditional sentence, which describes a real or highly probable situation. For facts and habits like this one, Italian simply uses the present indicative tense in both halves of the sentence: ci sono (there are) and prescrive (prescribes).

Since sintomi is masculine plural, does gravi end in -i to match its gender?

It matches the plural number, but not specifically the masculine gender. Grave (serious) is an adjective that ends in -e in the singular. Adjectives that end in -e change to -i for the plural for both masculine and feminine nouns.