**Core Concept**
The question is testing the use of antiepileptic drugs for the treatment of neuropathic pain conditions such as postherpetic neuralgia and diabetic neuropathy. These conditions involve chronic pain due to nerve damage, and antiepileptic drugs have been found to be effective in managing such pain.

**Why the Correct Answer is Right**
Antiepileptic drugs like gabapentin and pregabalin have been shown to be effective in treating neuropathic pain conditions. They work by modulating the activity of voltage-gated calcium channels in the nervous system, which reduces the release of pain-causing neurotransmitters such as glutamate and substance P. Gabapentin and pregabalin are commonly used for the treatment of postherpetic neuralgia and diabetic neuropathy, making them a suitable choice for agent 'A'.

**Why Each Wrong Option is Incorrect**
* **Option B:** This option is incorrect because lamotrigine is primarily used as an antiepileptic drug for the treatment of bipolar disorder and is not commonly used for the treatment of postherpetic neuralgia or diabetic neuropathy.
* **Option C:** This option is incorrect because topiramate is an antiepileptic drug that is used for the treatment of epilepsy and migraines, but it is not typically used for the treatment of neuropathic pain conditions.
* **Option D:** This option is incorrect because valproate is an antiepileptic drug that is used for the treatment of epilepsy and bipolar disorder, but it is not commonly used for the treatment of postherpetic neuralgia or diabetic neuropathy.

**Clinical Pearl / High-Yield Fact**
Gabapentin and pregabalin are often used as a first-line treatment for neuropathic pain conditions due to their efficacy and relatively favorable side effect profile. However, they should be used with caution in patients with renal impairment, as they are primarily excreted by the kidneys.

**Correct Answer: C. Pregabalin.**