Okay, let's tackle this question. The patient is 40 years old with arthritis affecting the PIP (proximal interphalangeal), DIP (distal interphalangeal), and carpometacarpal joint of the thumb. The wrist and metacarpophalangeal joints are spared. The question is asking for the most likely diagnosis.

First, I need to recall the different types of arthritis and their typical joint involvement. Osteoarthritis (OA) often affects weight-bearing joints like the knees, hips, spine, and the DIP and carpometacarpal joints of the thumb. It's more common in older adults, but can occur in younger individuals with risk factors. Rheumatoid arthritis (RA) typically presents with symmetric involvement of small joints, especially the metacarpophalangeal (MCP) and proximal interphalangeal joints, and often affects the wrist. The pattern described here is asymmetric and spares the MCP and wrist, which is less typical for RA. Psoriatic arthritis can affect DIP joints (called "sausage toes" if multiple are involved), but it's often associated with skin psoriasis. Gout and pseudogout can affect various joints but don't have this specific pattern.

The key points here are the involvement of DIP and carpometacarpal joints, which are classic for OA. RA usually involves the MCP and PIP joints symmetrically, which are not mentioned here. Psoriatic arthritis might involve DIPs but is usually associated with skin findings. The sparing of wrist and MCP joints makes RA less likely. So the most probable diagnosis is osteoarthritis.

**Core Concept**
This question tests knowledge of **joint involvement patterns in degenerative arthritis**. Osteoarthritis (OA) characteristically affects **distal interphalangeal (DIP)**, **carpometacarpal (CMC)** of the thumb, and **proximal interphalangeal (PIP)** joints in a **"Heberden's and Bouchard's nodes"** pattern, while sparing **metacarpophalangeal (MCP)** and **wrist** joints.

**Why the Correct Answer is Right**
Osteoarthritis arises from **cartilage degeneration** and subchondral bone remodeling. The described joint distribution (DIP, PIP, CMC) aligns with **primary OA of the hand**, which is **asymmetric** and **non-inflammatory**. The absence of synovitis or systemic features (e.g., morning stiffness >30 minutes) further supports OA over inflammatory arthritides.

**Why Each Wrong Option is Incorrect**
**Option A:** Rheumatoid arthritis (RA) involves **symmetric MCP/PIP/wrist** joints and is associated with **rheumatoid factor positivity** and **synovitis**. Wrist involvement is almost universal in RA.
**Option B:** Psoriatic arthritis (PsA) may affect **DIPs** (salmon patches) but is typically **associated with psoriasis** and **enthesitis**. It often involves **MCP joints** and **dactylitis**.
**Option C:** Gout/pseudogout typically presents with **acute monoarticular attacks** in **first MTP

✓ Correct Answer: B. Osteoarthritis