Differential Diagnosis for Localized Tenderness on the Left Posterior Thigh

Single Most Likely Diagnosis

• Piriformis Syndrome: This condition is characterized by pain and tenderness in the buttock and posterior thigh, often due to compression of the sciatic nerve by the piriformis muscle. The symptoms worsen with sitting, which increases pressure on the nerve, making it a likely cause for the described symptoms.

Other Likely Diagnoses

• Hamstring Strain: A strain in the hamstring muscles, which are located at the back of the thigh, can cause localized tenderness and pain, especially if the strain is near the origin of the muscles under the buttock. Sitting can exacerbate the strain, leading to increased pain.
• Gluteal Strain: Similar to a hamstring strain, a strain in the gluteal muscles can cause pain and tenderness in the area, including the posterior thigh just below the buttock. Activities like sitting can irritate the injured muscle, worsening the symptoms.
• Sciatica: While often associated with radiating pain down the leg, sciatica can also present with localized pain and tenderness in the posterior thigh, especially if the sciatic nerve is irritated near its origin under the buttock. Sitting can exacerbate sciatic pain.

Do Not Miss Diagnoses

• Deep Vein Thrombosis (DVT): Although less common, DVT should be considered, especially if there are risk factors such as recent travel, surgery, or immobilization. DVT can cause localized tenderness and pain, and while it's less likely to be directly related to sitting, it's a critical diagnosis not to miss due to its potential for severe complications.
• Abscess or Infection: An abscess or soft tissue infection in the buttock or thigh area can cause localized pain and tenderness. Sitting might increase discomfort due to pressure on the affected area. These conditions require prompt medical attention.

Rare Diagnoses

• Ischial Bursitis: Inflammation of the bursa located under the ischial tuberosity (sitting bone) can cause pain and tenderness in the area, which might radiate to the posterior thigh. This condition is less common but could be considered, especially if other diagnoses are ruled out.
• Osteitis or Bone Tumor: Rarely, localized tenderness could be due to an inflammatory process (osteitis) or a tumor of the bone in the affected area. These conditions would typically have additional symptoms and might not directly correlate with sitting, but they are important to consider in a comprehensive differential diagnosis.