Cardiac muscle biopsied near the ventricular apex shows pale-staining, enlarged fibers with abundant sarcoplasm, few peripheral myofibrils, and numerous glycogen granules, connected by many gap junctions. These modified cardiac fibers constitute which conducting structure?

• A Nodal cardiomyocytes of the sinoatrial node
• B Purkinje fibers of the subendocardial conducting network ✓
• C Transitional cells of the atrioventricular bundle trunk
• D Working ventricular myocytes undergoing hypertrophy

Explanation

Purkinje fibers are larger than working cardiomyocytes, stain pale due to abundant glycogen and sparse myofibrils concentrated peripherally, and lie just beneath the endocardium. Their profuse gap junctions permit rapid impulse spread. Working hypertrophied myocytes retain central nuclei and dense myofibrils, eliminating option D.

Reference: Junqueira's Basic Histology: Text and Atlas, 16th ed.

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