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Difficulty: MediumCellular Injury and Adaptation

A 32-year-old man presents to the physical therapy clinic for his initial evaluation following a right knee anterior cruciate ligament (ACL) reconstruction. For the past four weeks, his right lower extremity has been immobilized in a rigid extension brace, and he has been strictly non-weight-bearing. On physical examination, the surgical incisions are well-healed without erythema or discharge. However, the circumference of his right mid-thigh is 4.5 cm less than that of his left mid-thigh. He reports no pain or paresthesias in the limb. Which of the following intracellular processes is primarily responsible for the macroscopic changes observed in this patient's right lower extremity?

  1. Covalent attachment of ubiquitin to cytoskeletal proteinsAnswer
  2. B
    Release of cytochrome c from the mitochondrial intermembrane space
  3. C
    Massive influx of extracellular calcium across the plasma membrane
  4. D
    Transformation of mature skeletal muscle cells into adipocytes
  5. E
    Failure of the ATP-dependent sodium-potassium pump

Answer

The correct answer is the covalent attachment of ubiquitin to cytoskeletal proteins, which facilitates targeted protein degradation.
The patient's clinical presentation of muscle wasting after prolonged casting is a classic example of disuse atrophy. At the cellular level, atrophy is characterized by a reduction in the size of the cell to achieve a new equilibrium with a reduced workload or nutrient supply. This shrinkage is achieved through decreased protein synthesis and increased protein degradation. The primary mechanism for degrading intracellular proteins in this context is the ubiquitin-proteasome pathway. Nutrient deficiency or disuse activates ubiquitin ligases, which attach ubiquitin molecules to target cytoskeletal proteins. These 'tagged' proteins are then recognized and degraded by the proteasome.

Step-by-Step Solution

1
Analyze the clinical presentation and physical exam findings.
The patient has a 4.5 cm reduction in thigh circumference after four weeks of immobilization, indicating profound muscle wasting.
Establishing the clinical diagnosis of disuse atrophy is the first step before determining the subcellular mechanism.
2
Determine the cellular adaptation taking place.
The skeletal muscle cells have undergone atrophy (shrinkage in cell size), rather than necrosis or apoptosis.
Matching the clinical sign (muscle wasting without inflammation or cell death) to the correct adaptive response narrows down the potential molecular pathways.
3
Identify the primary molecular mechanism of cellular atrophy.
Atrophy occurs via decreased protein synthesis and increased protein degradation, primarily driven by the ubiquitin-proteasome pathway.
This directly answers the stem's question regarding the intracellular process responsible for the macroscopic changes.

Key Concept

Disuse atrophy of skeletal muscle is a reversible cellular adaptation characterized by decreased cell size, primarily mediated by increased degradation of cytoskeletal proteins via the ubiquitin-proteasome pathway.
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Topics

Cellular Injury and Adaptation

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