**Core Concept:**
Internal hordeolum, also known as an stye, is a localized inflammatory condition involving the meibomian gland, which is one of the major glands responsible for producing the tear film. The meibomian gland is located within the eyelid, specifically in the tarsal plate. In internal hordeolum, the meibomian gland becomes infected, leading to redness, swelling, and pain in the affected area.

**Why the Correct Answer is Right:**
In the context of internal hordeolum, the most affected gland is the meibomian gland due to its location and function. The meibomian gland is an essential part of the tear film production and plays a crucial role in maintaining the ocular surface health. When the meibomian gland becomes infected, it results in inflammation and swelling, which presents as an internal hordeolum.

**Why Each Wrong Option is Incorrect:**
A. The lacrimal gland (Option A) is responsible for producing the aqueous layer of the tear film and is not directly involved in internal hordeolum. It is located in the lacrimal sac area, which is distinct from the location of the meibomian gland.
B. The gland of Zeis (Option B) is another eyelid gland that contributes to tear film production. However, it is not as critical as the meibomian gland and does not typically cause internal hordeolum.
C. The sebaceous gland (Option C) is responsible for producing the lipid layer of the tear film but is also not directly involved in internal hordeolum. These glands are located throughout the body and not specifically in the eyelid area.
D. The submandibular gland (Option D) is a major salivary gland located in the submandibular region, far removed from the site of internal hordeolum. This gland is not directly related to tear film production or eyelid inflammation.

**Clinical Pearl:**
Internal hordeolum is primarily caused by an infection of the meibomian gland, which is responsible for producing the lipid layer of the tear film. Infections can be bacterial (most commonly Staphylococcus aureus) or fungal, leading to inflammation and the characteristic presentation of an internal hordeolum. Early diagnosis and treatment is crucial to prevent complications such as rupture and spread of infection.