Trigger Finger (Stenosing Tenosynovitis): This condition is characterized by tenderness and stiffness, particularly in the morning, with difficulty flexing the finger due to inflammation of the tendon sheath. The middle finger is commonly affected, and the symptoms match the description provided.

Osteoarthritis (OA): Although OA typically affects multiple joints and is more common in older adults, it can present with tenderness and limited mobility in the affected joints, including the PIP and MCP joints of a single finger.
Rheumatoid Arthritis (RA): Early stages of RA might present with localized joint inflammation and stiffness, although it usually involves multiple joints and has systemic symptoms.
Trauma or Sprain: Direct injury to the finger can cause localized tenderness and difficulty with flexion due to swelling, bruising, or ligament sprain.

Infectious Arthritis: Bacterial or fungal infections within the joint can cause severe pain, swelling, and limited mobility. Although less common, missing this diagnosis could lead to serious complications, including joint destruction and systemic infection.
Gout or Pseudogout: These conditions can cause sudden, severe joint pain and swelling, typically affecting a single joint at a time. Missing these diagnoses could lead to inadequate treatment and recurrent episodes.

Scleroderma: A chronic autoimmune disease that can cause skin thickening and joint involvement, leading to limited mobility. However, it usually presents with systemic symptoms and skin changes.
Psoriatic Arthritis: A form of arthritis associated with psoriasis, which can cause joint pain, swelling, and stiffness. It might affect a single joint but is often accompanied by skin and nail changes.
Tendinous or Ligamentous Cysts: Ganglion cysts or other types of cysts can form near the joints, causing localized pain and limited mobility due to compression of the surrounding tissues.