**Core Concept**
Pressure sores, also known as decubitus ulcers, are a type of wound that occurs due to prolonged pressure on the skin and underlying tissues. They can be classified into four stages based on their depth and extent of tissue involvement, with each stage indicating a more severe level of tissue damage.

**Why the Correct Answer is Right**
The correct answer, **Stage 3**, indicates that the pressure sore has extended into the subcutaneous tissue but underlying structures are not involved. This stage is characterized by a full-thickness loss of the skin, exposing the subcutaneous tissue. The sore is typically deep and can have a base that is covered in yellow or gray slough. The subcutaneous tissue may be visible, but the underlying fascia, muscle, or bone is still intact.

**Why Each Wrong Option is Incorrect**
**Option A:** Stage 1 pressure sores are superficial and limited to the epidermis and dermis, without any visible damage. Therefore, they do not extend into the subcutaneous tissue.

**Option B:** Stage 2 pressure sores involve a partial-thickness loss of the skin, exposing the dermis. They may appear as an open wound, but the subcutaneous tissue is not involved.

**Option D:** Stage 4 pressure sores are the most severe and involve a full-thickness loss of the skin and underlying tissues, including the fascia, muscle, or bone. This stage indicates that the sore has extended beyond the subcutaneous tissue.

**Clinical Pearl / High-Yield Fact**
It is essential to regularly assess and reposition patients at risk of developing pressure sores to prevent progression to a more severe stage. A pressure-redistributing mattress or cushion can also help reduce the risk of developing a pressure sore.

**✓ Correct Answer: C. Stage 3**