**Core Concept**
A stye is a type of localized infection or inflammation of the eyelid margin, typically involving the meibomian glands or the hair follicles of the eyelashes. It is characterized by the formation of a painful, tender nodule on the eyelid.

**Why the Correct Answer is Right**
Hordeolum externum, also known as external hordeolum or stye, is a type of stye that occurs on the outside of the eyelid, typically involving the hair follicles of the eyelashes. It is caused by the introduction of bacteria, such as Staphylococcus aureus, into the hair follicle, leading to inflammation and infection. The meibomian glands, which are located inside the eyelid, are involved in the formation of meibum, a type of sebum that helps to lubricate the eye. In contrast, Hordeolum internum affects the meibomian glands within the eyelid.

**Why Each Wrong Option is Incorrect**
**Option A:** Chalazion is a type of stye that occurs inside the eyelid, involving the meibomian glands. It is caused by blockage of the meibomian gland, leading to inflammation and granuloma formation.
**Option B:** Hordeolum internum, also known as internal hordeolum, is a type of stye that occurs inside the eyelid, involving the meibomian glands. It is also caused by bacterial infection, but is distinct from external hordeolum.
**Option D:** While Hordeolum externum is commonly referred to as a stye, both Hordeolum internum and Chalazion are also types of styes, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to differentiate between Hordeolum externum and Hordeolum internum, as the treatment approach and outcome may vary. Hordeolum externum typically responds well to warm compresses and topical antibiotics, whereas Hordeolum internum may require incision and drainage.

✓ Correct Answer: C. Hordeolum externum