**Core Concept**
Type-II lepra reaction, also known as erythema nodosum leprosum (ENL), is a systemic immune complex-mediated hypersensitivity reaction that occurs in lepromatous leprosy patients. It is characterized by the formation of immune complexes and their deposition in various tissues, leading to inflammation and tissue damage.

**Why the Correct Answer is Right**
Type-II lepra reaction is caused by the deposition of immune complexes containing leprosy bacilli and host antibodies in the tissues. This leads to the activation of the complement system, resulting in the formation of C3a and C5a, which are potent chemotactic agents that attract neutrophils and macrophages to the affected area. The presence of these immune complexes and the subsequent inflammatory response are responsible for the clinical manifestations of ENL, including fever, skin lesions, and joint inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ENL is not typically associated with tuberculoid leprosy, where the immune response is more cellular and less likely to lead to the formation of immune complexes.
**Option B:** This option is incorrect because ENL is not a characteristic feature of borderline leprosy, although it can occur in this form of the disease.
**Option C:** This option is incorrect because ENL is not a feature of lepromatous leprosy alone, but rather a complication that can occur in any form of leprosy with a high bacterial load.

**Clinical Pearl / High-Yield Fact**
ENL can be triggered by various factors, including the initiation of treatment with corticosteroids, which can lead to the release of immune complexes from the tissues.

**Correct Answer: D. Lepromatous leprosy**