A middle-aged woman presents with intermittent unilateral pain in the submandibular region. What is the most probable cause?

The most probable cause of intermittent unilateral pain in the submandibular region in this case is sialolithiasis, which is typically caused by the presence of a calculus, or stone, in the salivary duct. These calcifications can obstruct the flow of saliva, leading to swelling and pain, especially when the affected gland attempts to produce saliva, which often happens during eating.

In the submandibular gland, the duct is longer and has a more complex pathway than the other salivary glands, making it more prone to stone formation. This condition is common and often results in episodes of pain that may resolve if the stone passes or the obstruction is relieved. This character of intermittent pain fits well with the provided symptoms of the patient.

Other options, while they represent potential conditions that can occur in the submandibular region, do not fit as closely with the symptom profile of unilateral pain. A ranula, which is a type of mucocele, typically presents as a swelling rather than causing specific pain episodes. Cysts can also appear as painless swellings. Lastly, a mucocele is often associated with minor salivary glands and typically has a different presentation than what would be expected in this scenario.