**Core Concept**
The stretch reflex is a monosynaptic reflex arc that responds to muscle stretch by generating a contraction. It is mediated by sensory neurons directly synapsing with motor neurons in the spinal cord, making it a fundamental example of spinal reflex integration.

**Why the Correct Answer is Right**
The stretch reflex is initiated when muscle stretch activates muscle spindles, which send signals via Ia afferents to the spinal cord. These signals synapse directly with alpha motor neurons in the same segment, resulting in muscle contraction. This reflex arc operates independently of higher centers and is entirely localized in the spinal cord, particularly in the ventral horn. The spinal cord is the lowest and most primitive level of integration for this reflex.

**Why Each Wrong Option is Incorrect**
Option A: The cerebral cortex does not initiate or integrate the stretch reflex; it modulates motor output but is not involved in the basic reflex arc.
Option B: The lower medulla is involved in respiratory and some autonomic reflexes, not stretch reflexes.
Option D: "All of the above" is incorrect because only the spinal cord directly mediates the stretch reflex; higher centers do not integrate it.

**Clinical Pearl / High-Yield Fact**
The stretch reflex is a key clinical sign in neurological exams—its absence or hyperactivity indicates spinal cord or motor neuron pathology. The monosynaptic nature of this reflex makes it the simplest and most basic reflex in the nervous system.

✓ Correct Answer: C. Spinal cord