**Core Concept**
Thin filaments in skeletal and cardiac muscle are composed of actin, tropomyosin, and troponin. These proteins work together to regulate muscle contraction by controlling the access of myosin to actin binding sites. Myosin is part of the thick filament, not the thin filament.

**Why the Correct Answer is Right**
Myosin is the primary component of the thick filament, not the thin filament. Thin filaments consist of actin (the globular subunit), tropomyosin, and troponin, which regulate contraction by blocking myosin-binding sites on actin until calcium binds to troponin. Myosin forms the thick filament and interacts with actin during cross-bridge cycling. Thus, myosin is absent from the thin filament structure.

**Why Each Wrong Option is Incorrect**
Option A: Actin is a key structural protein in thin filaments and forms the backbone of the filament.
Option B: Troponin is a regulatory protein bound to actin in the thin filament, essential for calcium-dependent contraction.
Option D: Tropomyosin covers the actin-binding sites in the thin filament and shifts upon calcium binding to expose actin for myosin interaction.

**Clinical Pearl / High-Yield Fact**
Remember: "Thin filament = Actin + Tropomyosin + Troponin; Thick filament = Myosin." Myosin is the motor protein that pulls on actin during contraction — it is never part of the thin filament.

✓ Correct Answer: C. Myosin