**2. Etiology**

PUK may occur because of a variety of ocular and systemic disorders, including infectious and noninfectious conditions. Understanding the following causes of PUK is important for physicians, as PUK can be a rare manifestation of a common disease as well as a common manifestation of a rare disorder. Nearly half of all noninfectious PUK cases are associated with connective tissue diseases, most commonly rheumatoid arthritis (RA). RA is associated with 34% of noninfectious PUK cases; in 50%, it occurs bilaterally and appears in the later stages of the disease [11]. When associated with vasculitis, such as granulomatosis with polyangiitis (GPA), PUK is more often observed as the first manifestation of the underlying condition [12]. Studies suggest that infections are the second most common cause of PUK (about 20% of all cases); therefore, it is essential to rule out probable infectious etiology before starting any immunomodulatory therapy [13].

#### **2.1 Local causes**

a. Infectious

