In human skeletal muscles, when does the condition known as atrophy occur?

Introduction / Context:
Muscles can change in size and function in response to activity, inactivity, disease, and injury. Two important terms are hypertrophy, which refers to an increase in muscle size, and atrophy, which refers to a decrease in muscle size. This question tests your understanding of the definition of atrophy and how it differs from other muscle related conditions.

Given Data / Assumptions:

• The focus is on skeletal muscle, not smooth or cardiac muscle.
• The options describe different scenarios involving waste accumulation, increased use, and tendon injury.
• We assume standard physiological definitions of atrophy and hypertrophy.

Concept / Approach:
Atrophy is defined as a reduction in the size and functional capacity of a tissue or organ, often due to disuse, loss of nerve supply, decreased blood supply, malnutrition, or disease. In skeletal muscle, atrophy typically involves thinning of fibers and reduced strength. It is distinct from temporary fatigue due to waste buildup and from hypertrophy, which refers to increased muscle size with training. Tendon injuries involve connective tissues and are not themselves atrophy, although they can lead to disuse atrophy later.

Step-by-Step Solution:
Step 1: Examine option A. It states that atrophy occurs when muscle decreases in size and function due to disease or trauma. This matches the definition of atrophy as a wasting away or shrinkage of muscle.Step 2: Examine option B. Accumulation of carbon dioxide and lactic acid relates more to muscle fatigue during acute exercise, not long term shrinkage of muscle tissue.Step 3: Examine option C. Increased use of skeletal muscle causing an increase in strength describes hypertrophy, the opposite of atrophy.Step 4: Examine option D. Stretching or injuring tendons describes strain or tendinitis, not directly the definition of atrophy, although it might indirectly lead to atrophy due to disuse.Step 5: Conclude that option A correctly defines the condition of atrophy in skeletal muscles.

Verification / Alternative check:
You can verify the correct option by recalling that the prefix "a" often indicates loss or absence, and "trophy" relates to nourishment or growth. Together they imply a loss of nourishment or growth, matching the idea of muscles shrinking due to disuse or disease. Medical descriptions of muscle atrophy consistently emphasize reduced muscle mass and decreased functional ability.

Why Other Options Are Wrong:
Option B is wrong because it describes metabolic by product buildup during exercise, leading to temporary fatigue or soreness, not lasting muscle shrinkage. Option C is wrong because it describes hypertrophy, an adaptive increase in muscle size with training. Option D is wrong because it focuses on tendon injury, which is a connective tissue problem, not directly the definition of atrophy of muscle fibers.

Common Pitfalls:
One common mistake is to confuse atrophy with muscle fatigue, believing that any negative change in muscle function after exercise is atrophy. Another pitfall is mixing up hypertrophy and atrophy because the words sound similar. To avoid this, remember that hypertrophy involves growth and increased size, whereas atrophy refers to wasting or shrinking of a tissue or organ.

Final Answer:
When muscle decreases in size and function because of disease, disuse, or trauma.