**Question:** Which of the following is not a cause for interstitial keratitis?

Core Concept: Interstitial keratitis is an inflammation of the cornea that affects the stroma, and is characterized by pain, photophobia, and reduced visual acuity. Various causes can lead to this condition, which must be identified to guide appropriate treatment.

**Core Concept Explanation:** Interstitial keratitis can result from infections, autoimmune diseases, and medications. Infections include herpetic keratitis, caused by herpes simplex virus, and fungal keratitis, caused by various fungi. Autoimmune diseases such as rheumatoid arthritis, scleritis, and Behçet's disease can also induce keratitis. Drug-induced interstitial keratitis is due to medications like minocycline, methotrexate, and cyclosporine.

**Why the Correct Answer is Right:** Option D, "HIV infection," is not mentioned as a cause for interstitial keratitis in the provided options. While HIV can cause ocular manifestations, it generally presents as herpetic keratitis due to Herpes simplex virus reactivation or as atypical infections like cryptococcosis and toxoplasmosis.

**Why Each Wrong Option is Incorrect:**

A. Herpetic keratitis: This is caused by the herpes simplex virus and not a specific cause for interstitial keratitis.
B. Fungal keratitis: This is caused by various fungi and not relevant to the question's focus on interstitial keratitis.
C. HIV infection: While HIV can cause ocular manifestations, it typically presents as herpetic keratitis due to Herpes simplex virus reactivation or as atypical infections like cryptococcosis and toxoplasmosis.

**Clinical Pearl:** Correct identification of the etiology is crucial for appropriate treatment and management of interstitial keratitis. Adequate knowledge of different causes and their specific presentations can help in formulating an accurate diagnosis and initiating the right therapy.

**Correct Answer:** D. HIV infection

**Explanation:** HIV infection is not explicitly mentioned as a cause for interstitial keratitis in the provided options. Interstitial keratitis is primarily caused by infections with herpes simplex virus (herpetic keratitis) or atypical infections like cryptococcosis and toxoplasmosis. Understanding the various causes of interstitial keratitis is essential for accurate diagnosis and appropriate treatment.