De Quervain's Tenosynovitis: This condition, which affects the tendons on the thumb side of your wrist, is a common cause of pain at the base of the thumb. It is often associated with repetitive movements or direct injury to the area, making it a likely diagnosis for acute pain in this location.

Thumb Sprain: A sprain of the thumb, particularly the ulnar collateral ligament, can cause acute pain at the base of the thumb. This is often due to a sudden injury, such as a fall onto the thumb.
Osteoarthritis of the First Carpometacarpal Joint: While more commonly associated with chronic pain, an acute flare of osteoarthritis in the joint at the base of the thumb can cause significant pain.
Ganglion Cyst: A ganglion cyst that forms near the base of the thumb can cause pain, especially if it presses on nearby nerves or tendons.

Infection (e.g., Cellulitis or Abscess): An infection in the soft tissues at the base of the thumb can cause acute pain and swelling. It's crucial to identify and treat infections promptly to prevent serious complications.
Fracture (e.g., Scaphoid or First Metacarpal Fracture): A fracture in the bones near the base of the thumb can cause acute pain. Missing a fracture can lead to improper healing and long-term dysfunction.
Gout or Pseudogout: These conditions can cause sudden, severe pain in the joints, including the base of the thumb. While less common, they are important to consider due to their distinct treatment approaches.

Giant Cell Tumor of the Tendon Sheath: A rare, benign tumor that can cause pain and swelling at the base of the thumb if it develops in this area.
Sarcoidosis: This systemic disease can rarely cause granulomatous inflammation in the tendons or joints at the base of the thumb, leading to pain.
Tuberculosis or Other Uncommon Infections: Though rare, tuberculosis or other unusual infections can affect the bones or joints at the base of the thumb, causing acute pain.