Sjögren's Syndrome: This autoimmune disorder is characterized by exocrine gland dysfunction, leading to symptoms like xerostomia (dry mouth). It can also cause arthralgia (joint pain) and, less commonly, neurological symptoms such as myoclonus (sudden, involuntary muscle jerks). The combination of these symptoms makes Sjögren's Syndrome a strong candidate for the single most likely diagnosis.

Lupus: Systemic Lupus Erythematosus (SLE) is an autoimmune disease that can cause a wide range of symptoms, including arthralgia, neurological manifestations (which could include myoclonus), and dry mouth. Its broad symptom profile makes it a plausible diagnosis.
Rheumatoid Arthritis (RA): While primarily known for causing joint inflammation and pain (arthralgia), RA can also have systemic manifestations, including dry mouth (xerostomia) due to secondary Sjögren's Syndrome. Neurological symptoms like myoclonus are less common but can occur, especially in more severe or advanced cases.
Medication Side Effects: Certain medications, particularly those used to treat psychiatric conditions or seizures, can cause myoclonus and dry mouth as side effects. If the patient is on any new medications, this could be a likely explanation.

Lyme Disease: In its later stages, Lyme disease can cause neurological symptoms (neuroborreliosis), including myoclonus, and can also lead to arthralgia. Dry mouth might be less directly related but could occur due to systemic illness. Missing this diagnosis could lead to significant morbidity.
HIV: Early stages of HIV infection can present with a variety of nonspecific symptoms, including arthralgia, myoclonus (due to neurological involvement), and dry mouth. Given the potential severity of untreated HIV, it's crucial not to miss this diagnosis.

Whipple's Disease: A rare, systemic bacterial infection that can cause a wide range of symptoms, including arthralgia, neurological symptoms like myoclonus, and dry mouth. It's essential to consider this, although it's rare, due to its treatable nature with antibiotics.
Paraneoplastic Syndromes: These are rare disorders that are triggered by an altered immune system response to a neoplasm. They can cause a variety of symptoms, including myoclonus, arthralgia, and potentially dry mouth, depending on the tumor and the immune response involved.