**Core Concept**
Reiter's syndrome, also known as reactive arthritis, is a type of seronegative spondyloarthropathy that occurs in response to an infection, often urinary tract infection, gastrointestinal infection, or sexually transmitted infection. It is characterized by the classic triad of conjunctivitis, urethritis, and arthritis.

**Why the Correct Answer is Right**
The patient's symptoms of gritty pain in the eye and joint pain following an episode of urinary infection are classic manifestations of Reiter's syndrome. This condition is thought to occur due to an autoimmune response triggered by the infection, leading to inflammation in the joints, eyes, and other tissues. The exact mechanism is not fully understood, but it is believed to involve the activation of immune cells and the release of pro-inflammatory cytokines, such as TNF-alpha and IL-1 beta.

**Option A:** This option is incorrect because it is the correct answer.
**Option B:** Behçet's syndrome, also known as Behçet's disease, is a form of vasculitis that affects the blood vessels and is characterized by symptoms such as oral aphthous ulcers, genital ulcers, and uveitis, which are not present in this patient.
**Option C:** Sarcoidosis is a systemic granulomatous disease that can affect any organ, but it typically presents with pulmonary symptoms and skin lesions, which are not described in this patient.
**Option D:** Systemic Lupus Erythematosus (SLE) is a chronic autoimmune disease that can affect multiple organs, but it typically presents with a wide range of symptoms, including fever, fatigue, and joint pain, and is associated with specific autoantibodies, such as ANA and anti-dsDNA, which are not mentioned in this patient.

**Clinical Pearl / High-Yield Fact**
Reiter's syndrome often follows an infection, and the symptoms can be quite nonspecific, making it a classic exam trap to consider other diagnoses first. Remember the classic triad of conjunctivitis, urethritis, and arthritis, and consider this diagnosis in patients with a history of recent infection.

**✓ Correct Answer: A. Reiter's syndrome**