Deep tendon reflexes (DTRs), more accurately termed muscle stretch reflexes, are monosynaptic spinal reflex arcs testing the integrity of sensory (afferent) neurons, spinal cord segments, and motor (efferent) neurons. When the tendon is tapped, the muscle spindle within the muscle belly is stretched, activating Ia afferent sensory neurons that enter the dorsal root and synapse directly on alpha motor neurons in the ventral horn of the corresponding spinal cord segment. The alpha motor neuron fires, causing the muscle to contract (the reflex response). Simultaneously, Ia inhibitory interneurons inhibit the antagonist muscle (reciprocal inhibition). Upper motor neuron (UMN) lesions (stroke, MS, spinal cord compression above the reflex arc) remove descending inhibitory input, causing hyperreflexia (3+ to 4+), clonus, positive Babinski sign, and spasticity. Lower motor neuron (LMN) lesions (peripheral neuropathy, radiculopathy, anterior horn cell disease like ALS) damage the reflex arc itself, causing hyporeflexia or areflexia (0 to 1+), muscle atrophy, fasciculations, and flaccidity. The grading scale: 0 = absent, 1+ = diminished (may be normal), 2+ = normal, 3+ = brisk (may be normal in anxious patients), 4+ = clonus...