Basic Sciences: Anatomy, Physiology, Pathology, and Biomechanics
This domain tests the basic-science foundation behind clinical decisions: anatomy, physiology, pathology, and biomechanics. Questions may ask you to predict which nerve root is affected, explain tissue healing, or apply spinal coupling principles.
Core rules
- In the lumbar spine, a posterolateral disc herniation usually compresses the traversing root below the disc level: an L4–L5 herniation affects L5. In the cervical spine, the exiting root is affected at the disc level: a C5–C6 herniation affects C6.
- Localize by dermatome, myotome, and reflex: L5 - lateral leg and dorsum of foot, great-toe extension, no routine reflex; S1 - posterior calf and lateral foot, plantar flexion, Achilles reflex.
- Healing proceeds through hemostasis, inflammation, proliferation, and remodeling. Early inflammation is not a reason to force stretched tissue; controlled loading supports remodeling.
- Fryette’s laws describe coupling: in neutral mechanics, Type I, sidebending and rotation couple to opposite sides; in non-neutral flexion or extension, Type II, they couple to the same side. These are models, not universal rules for every region.
- Distinguish upper motor neuron signs, such as hyperreflexia, spasticity, and Babinski, from lower motor neuron signs, such as hyporeflexia, weakness, and atrophy.
Worked example
A patient has weakness of great-toe extension, sensory loss over the dorsum of the foot, and low back pain radiating into the lateral leg. The Achilles reflex is normal. The findings fit an L5 radiculopathy, commonly from an L4–L5 posterolateral disc herniation compressing the traversing L5 root. If the patient instead had a diminished Achilles reflex and weak plantar flexion, the localization would shift to S1. Reasoning from dermatome, myotome, and reflex prevents the common error of assigning the root by pain location alone.
Traps
Assigning the root level from the disc level without accounting for cervical versus lumbar anatomy. Confusing radiculopathy with myelopathy. Treating every positive orthopedic test as a disc lesion. Applying Fryette’s laws as if they were validated for every spinal region. Forgetting that imaging findings such as disc bulges are common in asymptomatic people.