**Core Concept**
The management of paranoid schizophrenia involves the use of antipsychotic medications to alleviate symptoms such as hallucinations and delusions. In young patients, particularly those with a thin build, the risk of metabolic side effects, including weight gain and dyslipidemia, is a significant concern.

**Why the Correct Answer is Right**
The best drug therapy for paranoid schizophrenia in a thin-built 21-year-old young male would be a second-generation (atypical) antipsychotic that has a lower risk of metabolic side effects. Risperidone is an example of such a medication. It is a dopamine D2 receptor antagonist that also has some serotonin 5-HT2A receptor antagonism, which contributes to its efficacy in treating psychotic symptoms with a lower risk of extrapyramidal side effects. Additionally, risperidone has a relatively lower risk of weight gain and metabolic disturbances, making it a suitable choice for young patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Haloperidol is a first-generation (typical) antipsychotic that is more likely to cause extrapyramidal side effects and metabolic disturbances, making it a less ideal choice for a young patient.
**Option B:** Olanzapine is a second-generation antipsychotic, but it has a higher risk of weight gain and metabolic disturbances, which may be a concern in a thin-built patient.
**Option C:** Quetiapine is another second-generation antipsychotic, but it has a higher risk of metabolic side effects and is not typically the first choice for paranoid schizophrenia.

**Clinical Pearl / High-Yield Fact**
When choosing an antipsychotic for young patients, consider the risk-benefit profile and the potential for metabolic side effects. Regular monitoring of weight, lipid profiles, and glucose levels is essential to mitigate these risks.

**Correct Answer:** C. Quetiapine is another second-generation antipsychotic, but it has a higher risk of metabolic side effects and is not typically the first choice for paranoid schizophrenia.