In a contracted skeletal muscle fiber,

Options

A. M line disappears
B. H zone elongates
C. I band remains constant
D. A band disappears

Answer

To solve the question regarding what happens in a contracted skeletal muscle fiber, we will analyze each of the options provided based on our understanding of muscle contraction.

### Step-by-Step Solution:

1. **Understanding Muscle Structure**:
- Skeletal muscle fibers are made up of units called sarcomeres, which contain thick (myosin) and thin (actin) filaments.
- The M line is the middle line of the sarcomere where thick filaments are anchored.
- The H zone is the area in the center of the A band where there are only thick filaments (myosin).
- The I band is the region that contains only thin filaments (actin).

2. **Muscle Contraction Mechanism**:
- During contraction, the thin filaments (actin) slide over the thick filaments (myosin).
- This sliding motion causes the sarcomere to shorten, which leads to muscle contraction.

3. **Analyzing Each Option**:
- **Option A: M line disappears**:
- As the actin slides past the myosin, the M line, which is the attachment site for thick filaments, effectively disappears during contraction. This is the correct answer.

- **Option B: H zone elongates**:
- The H zone does not elongate; instead, it remains the same or may even disappear as the actin filaments overlap with the myosin filaments during contraction. This option is incorrect.

- **Option C: I band remains constant**:
- The I band actually decreases in size during contraction as the actin filaments slide past the myosin. This option is also incorrect.

- **Option D: A band disappears**:
- The A band, which contains the thick filaments, does not disappear during contraction; it remains the same length. This option is incorrect.

4. **Conclusion**:
- Based on the analysis, the correct answer is that in a contracted skeletal muscle fiber, the **M line disappears**.

### Final Answer:
In a contracted skeletal muscle fiber, **M line disappears**.