**Core Concept**
The thick filament in muscle fibers is composed of a specific combination of proteins that provide structural support and facilitate contraction. The myosin heavy chain and myosin light chain are two essential components of the thick filament, while dystrophin and titin play distinct roles in muscle function.

**Why the Correct Answer is Right**
The correct answer, **C. 1 and 2**, is correct because the thick filament is primarily made up of the myosin heavy chain and myosin light chain. These two proteins work together to form the myosin head, which interacts with actin to produce muscle contraction. The myosin heavy chain provides the structural framework, while the myosin light chain regulates the activity of the myosin head.

**Option A:** This option is incorrect because dystrophin is not a component of the thick filament. Dystrophin is a cytoskeletal protein that plays a crucial role in maintaining muscle cell integrity and is associated with Duchenne muscular dystrophy.

**Option B:** This option is incorrect because titin is not a component of the thick filament. Titin is a giant sarcomeric protein that extends from the Z-disk to the M-line in the sarcomere and provides elasticity to the muscle.

**Option D:** This option is incorrect because while dystrophin and titin are both important muscle proteins, they are not components of the thick filament.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the thick filament is composed of myosin heavy chain and myosin light chain, which work together to facilitate muscle contraction. Understanding the composition of the thick filament is critical for diagnosing and treating muscle disorders.

**✓ Correct Answer: C. 1 and 2**