Rheumatoid Arthritis (RA): This is a common autoimmune disorder that often presents with symmetric polyarthralgia, particularly affecting the wrists, arms, shoulders, hands, and knees. The involvement of multiple joints, including one knee, aligns with the potential early presentation of RA, where not all joints may be symmetrically involved at the onset.

Osteoarthritis (OA): Although OA more commonly affects weight-bearing joints and the hands, it can also involve other joints, leading to polyarthralgia. The involvement of one knee could suggest OA, especially if there's a history of trauma, overuse, or age-related wear and tear.
Fibromyalgia: Characterized by widespread musculoskeletal pain accompanied by fatigue, sleep, memory, and mood issues. Fibromyalgia could be considered, especially if the pain is diffuse and not limited to the joints mentioned, and if there are other systemic symptoms.
Systemic Lupus Erythematosus (SLE): An autoimmune disease that can affect various parts of the body, including the joints, skin, kidneys, and other organs. Polyarthralgia is a common symptom, and SLE should be considered, especially in the presence of other systemic symptoms or signs.

Septic Arthritis: Although less likely given the polyarticular presentation, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. The involvement of a single knee could be a starting point, and if there's significant pain, redness, swelling, or systemic symptoms like fever, this diagnosis must be urgently considered.
Lyme Disease: Caused by the bacterium Borrelia burgdorferi, transmitted to humans through the bite of infected black-legged ticks. Lyme disease can cause arthritis, particularly in the knees, and should be considered, especially if there's a history of tick exposure or travel to endemic areas.
Gout or Pseudogout: These are forms of crystal-induced arthritis that can cause sudden, severe joint pain, typically affecting one joint at a time but can be polyarticular. They are important to consider, especially if there are sudden onset and severe pain in the affected joints.

Relapsing Polychondritis: A rare autoimmune disorder characterized by recurrent episodes of cartilage inflammation, which can affect various types of cartilage, including that found in the ears, nose, trachea, and joints.
Psoriatic Arthritis: A form of arthritis that affects some people who have psoriasis, characterized by joint pain, stiffness, and swelling. It's less likely if there's no known history of psoriasis, but it should be considered, especially if there are skin or nail changes.
Sjögren's Syndrome: An autoimmune disorder that primarily affects the glands that produce tears and saliva, but it can also cause joint pain and arthritis. It's a rare consideration but should be thought of, especially if there are symptoms of dry eyes or mouth.