Ulcus serpens is caused by
**Core Concept**
Ulcus serpens, also known as serpentine ulcer, is a type of corneal ulcer that affects the corneal epithelium and stroma. It is characterized by a serpentine or branching pattern of the ulceration. The underlying cause of ulcus serpens is often related to a bacterial infection, particularly with Pseudomonas aeruginosa.
**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of ulcus serpens. Pseudomonas aeruginosa is an opportunistic pathogen that can infect the cornea, especially in individuals with compromised corneal integrity. The bacterium produces enzymes such as elastase and protease, which contribute to the degradation of the corneal tissue and the formation of the serpentine ulcer. The corneal stroma, which is composed of keratocytes and collagen fibers, is particularly susceptible to damage from these enzymes.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a bacterium that is commonly associated with ulcus serpens. While other bacteria such as Staphylococcus aureus can cause corneal ulcers, they are not typically associated with the serpentine pattern seen in ulcus serpens.
**Option B:** This option is incorrect because it does not accurately describe the pathophysiology of ulcus serpens. Fungal keratitis is a different type of corneal infection that is caused by fungi such as Aspergillus and Fusarium.
**Option C:** This option is incorrect because it does not specify a bacterium that is commonly associated with ulcus serpens. While other bacteria such as Streptococcus pneumoniae can cause corneal ulcers, they are not typically associated with the serpentine pattern seen in ulcus serpens.
**Clinical Pearl / High-Yield Fact**
Ulcus serpens is often associated with contact lens wear, particularly with the use of soft contact lenses that are not properly cleaned or maintained. This is because Pseudomonas aeruginosa can colonize the contact lens and then infect the cornea.
**Correct Answer: D. Pseudomonas aeruginosa**