In a patient with widespread pain primarily in muscles and no joint swelling, what is the probable condition?

The condition characterized by widespread pain primarily in muscles accompanied by no joint swelling is most likely fibromyalgia. Fibromyalgia is a chronic pain disorder distinguished by its hallmark symptoms, which include widespread musculoskeletal pain, fatigue, sleep disturbances, and often co-occurring symptoms like cognitive difficulties and mood disorders.

In fibromyalgia, the pain is thought to be due to an amplified pain response to sensory signals in the brain. Unlike conditions such as rheumatoid arthritis, where joint inflammation and swelling are prominent, or osteoarthritis, which predominantly affects joint structures, fibromyalgia focuses on muscle pain without associated inflammatory joint features.

Chronic fatigue syndrome is noted for profound fatigue that doesn't improve with rest and can also include musculoskeletal pain, but the emphasis on widespread muscle pain without joint swelling aligns more closely with the fibromyalgia diagnosis. Thus, given the specific indicators of muscle pain and absence of joint symptoms, fibromyalgia emerges as the most probable condition for the patient described.