Step 1: Sort each option into its drug class first:
The question gives the class in brackets for three options. Fluoxetine is an SSRI, amitriptyline is a TCA and venlafaxine is an SNRI. Vilazodone has no class given.
That already hints that the odd one out is the answer.
Step 2: Know what "novel" means here:
Pharmacology books call a drug novel or atypical when it works by a mechanism different from the three big classes (TCA, SSRI, SNRI) or combines two actions.
Examples are vortioxetine, agomelatine, bupropion, mirtazapine and vilazodone.
Step 3: Look at mechanisms:
Fluoxetine: blocks the serotonin transporter only.
Amitriptyline: blocks serotonin and noradrenaline transporters, and also blocks muscarinic, histamine and alpha-1 receptors, which causes dry mouth, sedation and low blood pressure on standing.
Venlafaxine: blocks serotonin and, at higher doses, noradrenaline reuptake.
Vilazodone: blocks the serotonin transporter and is also a partial agonist at 5-HT1A receptors. This is a new combination.
Step 4: Conclusion from the mechanisms:
Three of the drugs fit the old standard groups. Only vilazodone has a mixed mechanism that sits outside them.
Step 5: Why the other three are not the answer:
Fluoxetine, amitriptyline and venlafaxine are all long established drugs that appear in every standard classification as members of their own groups. None of them is described as novel.
Step 6: Clinical note on vilazodone:
It is used for major depressive disorder in adults. Common side effects are nausea, diarrhoea, headache and insomnia. It must be taken with food, otherwise absorption falls.
Final Answer:
Vilazodone is the only drug in the list that falls outside the classical SSRI, TCA and SNRI groups. The answer is option (D).