**Core Concept**
An internal hordeolum, also known as a stye, is a localized infection of the eyelid margin, typically involving the meibomian gland or its duct. This condition is characterized by inflammation and purulent discharge, often accompanied by pain and swelling.

**Why the Correct Answer is Right**
The internal hordeolum occurs when the meibomian gland or its duct becomes obstructed, leading to bacterial overgrowth and subsequent infection. The most common causative organisms are Staphylococcus aureus and Streptococcus pneumoniae. The infection causes inflammation of the gland or duct, resulting in the characteristic symptoms of an internal hordeolum.

**Why Each Wrong Option is Incorrect**
**Option A:** The external hordeolum, also known as a stye, occurs on the outside of the eyelid, primarily affecting the eyelash follicle or the sebaceous glands of Zeis. It is not located on the inside of the eyelid.
**Option B:** The meibomian gland is located within the tarsal plate of the eyelid, not on the conjunctiva or the cornea.
**Option C:** The conjunctiva is a thin membrane covering the white part of the eye and the inside of the eyelids, but it is not the site of an internal hordeolum.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between an internal and external hordeolum, as the treatment approaches differ. An external hordeolum can be treated with warm compresses and topical antibiotics, whereas an internal hordeolum may require more aggressive treatment, including incision and drainage.

**Correct Answer:** D.