Brain & Spinal Cord
Unilateral loss of all pain and temperature sensation at and below the C5 spinal level is found to be due to an astrocytoma in the spinal cord at the C4 level on the contralateral side.
True or False: The axonal processes in the affected tract extend from cell bodies located in the contralateral dorsal horn.
TRUE
Loss of pain/temperature on one side below C5 from a contralateral C4 lesion reflects interruption of STT fibers that originated from dorsal horn neurons on the opposite side.
A 45-year-old construction worker undergoes surgical decompression for shoulder pain caused by entrapment beneath the superior transverse scapular ligament, which bridges the suprascapular notch. The suprascapular artery passes over this ligament, while a branch of the upper trunk of the brachial plexus travels beneath it to innervate two rotator cuff muscles.
True or False: The nerve that passes under this ligament innervates a muscle that assists with medial rotation at the glenohumeral joint.
FALSE
The suprascapular nerve (from the upper trunk, C5–C6) passes under the superior transverse scapular ligament to innervate the supraspinatus and infraspinatus, neither of which assist in medial rotation of the humerus.
A 24-year-old man falls on his outstretched hand and presents with wrist pain and swelling at the radiocarpal joint. The radius articulates directly with the scaphoid and lunate, while the ulna does not articulate with any carpal bones. One of those two carpal bones also articulates with the triquetrum, and an important artery passes superficial to the carpal bones on this side of the palm to contribute to the superficial palmar arterial arch.
True or False: This important artery (not the arch) gives rise to the common interosseous artery in the proximal forearm, which then gives rise to the posterior interosseous and anterior interosseous arteries, the main blood supply to the posterior compartment of the forearm and the deep flexor muscles of the anterior forearm, respectively.
TRUE
The ulnar artery runs on the same side of the palm as the triquetrum and gives rise to the common interosseous artery in the forearm, which divides into anterior and posterior interosseous branches supplying the deep flexor and extensor compartments of the forearm, respectively.
A 68-year-old woman with a long history of corticosteroid use presents with multiple vertebral compression fractures. Bone biopsy shows thin trabeculae with numerous Howship's lacunae containing multinucleated cells.
True or False: The cells in these lacunae are derived from the same lineage as macrophages.
TRUE
Osteoclasts occupy Howship’s lacunae and derive from the monocyte–macrophage lineage, sharing a hematopoietic origin with macrophages rather than osteoblasts.
A 20-week prenatal ultrasound reveals irregular vertebral spacing in a fetus with suspected congenital scoliosis. Each intervertebral disc is visible between adjacent vertebral bodies, with the central nucleus pulposus derived from the notochord and the surrounding annulus fibrosus formed from sclerotome mesenchyme. During normal development, sclerotomes re-segment so that each vertebra forms from the caudal half of one somite and the cranial half of the next, creating spaces for spinal nerves to emerge between the developing vertebrae.
True or False: The embryologic structure that gives rise to the nucleus pulposus directs sclerotome differentiation through fibroblast growth factor (FGF) signaling.
FALSE
The notochord induces sclerotome differentiation through Sonic Hedgehog (SHH) signaling, not FGF, guiding formation of vertebrae and the annulus fibrosus around the nucleus pulposus.
A 3-month-old infant presents with failure to thrive and developmental delay. Laboratory studies reveal megaloblastic anemia unresponsive to vitamin B12 and folate supplementation, and markedly elevated levels of orotic acid in the urine. The condition improves with oral uridine supplementation, which bypasses the metabolic block in pyrimidine synthesis.
True or False: The enzyme deficient in this syndrome is normally inhibited by hydroxyurea.
FALSE
Orotic aciduria results from UMP synthase deficiency, and hydroxyurea inhibits ribonucleotide reductase.
Fun fact: hydroxyurea is a first-line therapy for sickle cell anemia
A 32-year-old motorcyclist is thrown from his bike and lands on his upper back. On examination, there is localized tenderness and swelling over the most prominent spinous process in his neck.
True or False: Damage to the spinal cord segment at this vertebral level would result in sensory loss over the 3rd digit.
FALSE
Sensation over the 3rd digit corresponds to the C7 dermatome, which remains intact because the lesion involves the C8 segment—a relationship that is unique to the cervical spinal cord, where spinal nerves exit above their corresponding vertebrae until C8, after which numbering shifts to below.
A 45-year-old warehouse worker presents with upper back pain after years of heavy lifting. Imaging shows mild inflammation in the region deep to the rhomboid muscles. The affected muscle assists in elevating the ribs during inspiration and lies superficial to the erector spinae group. This muscle originates partly from the nuchal ligament, which connects the external occipital protuberance to the spinous processes of the cervical vertebrae.
True or False: This muscle is innervated by fibers that pass through the superior trunk of the brachial plexus.
FALSE
The serratus posterior superior muscle is innervated by an intercostal nerve branch rather than a nerve from the brachial plexus, despite its proximity to the dorsal scapular nerve, which supplies the rhomboids and levator scapulae. It lies deep to the rhomboids and superficial to the erector spinae, partly originating from the nuchal ligament along with the spinous processes of C7–T3.
A 28-year-old construction worker sustains a penetrating injury to his upper arm that damages fibers originating from the lateral cord of the brachial plexus. Weakness is noted in elbow flexion and forearm supination, and sensation is reduced over the lateral forearm. The affected nerve pierces the coracobrachialis muscle before continuing between the biceps brachii and brachialis to reach the forearm.
True or False: The affected nerve contains fibers from spinal levels C8–T1.
FALSE
The musculocutaneous nerve (C5–C7) arises from the lateral cord of the brachial plexus and provides motor innervation to the anterior arm and sensory innervation to the lateral forearm. C8-T1 fibers give rise to the ulnar nerve, but it arises from the medial cord of the brachial plexus, not the lateral, and would present with different symptoms.
A stab wound at the lateral base of a 27-year-old man's neck does not damage the subclavian artery but damages a nerve branching off the superior trunk of the brachial plexus that courses anteriorly and superficial over the subclavian artery to innervate a single muscle that attaches to one of the shoulder girdle bones which also happens to be the first bone in the body to begin ossification during embryogenesis.
True or False: This shoulder girdle bone forms from intramembranous ossification and thus forms bone directly from mesenchyme with no cartilaginous precursor.
TRUE
The clavicle is the first bone to ossify and forms by intramembranous ossification, in which bone develops directly from mesenchyme without a cartilaginous precursor. The muscle described is the subclavius, innervated by the nerve to subclavius (C5-C6), a branch of the superior trunk (C5–C6) of the brachial plexus. This same ossification mechanism also forms the flat bones of the skull (frontal, parietal) and portions of the mandible and maxilla.
Thalidomide exposure early in pregnancy can cause severe limb malformations such as phocomelia. During weeks 4–5 of development, limb buds normally form under the influence of signaling molecules secreted by a specialized thickened ectodermal ridge at each limb’s distal border. This ridge keeps the underlying mesenchyme proliferative and drives proximodistal limb outgrowth.
True or False: FGF is the signaling molecule produced by this ridge which promotes limb bud outgrowth.
TRUE
The apical ectodermal ridge (AER) secretes fibroblast growth factors (FGFs) that maintain proliferation of underlying mesenchyme and drive proximodistal limb outgrowth during weeks 4–5.
A 6-month-old infant presents with recurrent infections, chronic diarrhea, and failure to thrive. Laboratory studies confirm adenosine deaminase (ADA) deficiency, leading to severe combined immunodeficiency (SCID) caused by impaired degradation of adenosine and deoxyadenosine, which accumulate to toxic levels and inhibit lymphocyte proliferation.
True or False: The nitrogenous base of the nucleoside product of the ADA reaction differs from adenine because it is oxidized compared to adenine.
TRUE
ADA converts adenosine → inosine, replacing adenine with hypoxanthine, which is more oxidized than adenine.
A 70-year-old woman experiences a left-sided pure sensory stroke. MRI reveals damage to thalamocortical fibers projecting from a nucleus that relays body sensations to the postcentral gyrus. The infarct interrupts input from the spinothalamic and dorsal column/medial lemniscal tracts. Damage to which specific diencephalon-derived nucleus would explain this sensory loss?
Answer: Ventral posterolateral (VPL)
The ventral posterolateral nucleus of the thalamus receives somatosensory input from the spinothalamic and dorsal column/medial lemniscus pathways and relays it to the postcentral gyrus.
A 27-year-old man is involved in a car accident and sustains trauma to his right axilla. He presents with weakness when adducting and extending his arm at the glenohumeral joint, but no sensory loss in the upper limb. Examination reveals difficulty using the latissimus dorsi to pull his arm toward his torso, while deltoid and rotator cuff function remain intact. Which cord of the brachial plexus gives rise to the injured nerve?
Answer: Posterior cord
The thoracodorsal nerve (C6-C8) arises from the posterior cord of the brachial plexus and innervates the latissimus dorsi, which adducts and extends the humerus without conveying sensory fibers.
You notice the halal cart guy near HSB showing signs of weakness in the thenar muscles of his hand while cheffing it up. The innervation of these muscles is a branch of a nerve that courses alongside what vessel in the arm that the median cubital vein drains into?
Answer: Basilic vein
The median cubital vein connects the cephalic (lateral) and basilic (medial) veins at the cubital fossa, and it drains into the basilic vein, which continues proximally up the medial arm — the same side where the median nerve (whose branch supplies the thenar muscles) runs.
A 16-year-old boy with tall stature, long limbs, and lens dislocation is diagnosed with Marfan syndrome due to a mutation in the fibrillin-1 gene. His height results from increased linear growth at the epiphyseal plates caused by lax connective tissue support around the growth regions. Microscopic examination of cartilage from his external ear reveals normal chondrocytes embedded in lacunae but poor organization and thinning of the dense connective tissue layer surrounding the cartilage, which contains chondrogenic cells capable of differentiating into chondroblasts. Which specific histologic layer of the ear’s elastic cartilage is described?
Answer: Perichondrium
The perichondrium is the dense connective tissue layer surrounding elastic and hyaline cartilage (except articular cartilage), containing chondrogenic cells that differentiate into chondroblasts for cartilage growth and repair.
A 20-week prenatal ultrasound reveals a lumbosacral neural tube defect consistent with spina bifida. In the exposed spinal cord region, neurons fail to migrate beyond the zone that ependymal cells originate from, resulting in the other two zones failing to form. The outermost zone would normally give rise to what part of the spinal cord?
Answer: White matter
The outermost zone, called the marginal zone, of the developing spinal cord becomes white matter, containing myelinated axons from neurons whose cell bodies reside in the intermediate zone.
A 52-year-old man with a history of gout presents with severe pain and swelling of his right great toe, where examination reveals a firm subcutaneous tophus near the joint. In nucleotide metabolism, both the purine and pyrimidine synthesis pathways share an enzyme that activates ribose-5-phosphate from the pentose phosphate pathway into an energy-rich intermediate used to donate ribose-phosphate groups to nucleotide bases. Overactivity of this enzyme increases purine turnover, leading to excess uric acid production and deposition in joints. What is the name of this enzyme?
Answer: PRPP synthetase
PRPP synthetase converts ribose-5-phosphate from the pentose phosphate pathway to phosphoribosyl pyrophosphate (PRPP), fueling both purine and pyrimidine synthesis; its overactivity increases purine turnover and uric acid causing gout.
You witness a fellow medical student get thrown out of Morgan's at 1am after a few too many shots and start walking away unsteadily with an ataxic gait. Name two arteries that specifically supply the affected brain structure.
Answers: Posterior inferior cerebellar artery, anterior inferior cerebellar artery, superior cerebellar artery
The cerebellar arteries, part of the posterior circulation, supply the cerebellum, and alcohol-induced damage to the cerebellum can cause ataxic gait.
A 34-year-old man sustains a shoulder dislocation while playing basketball and presents with weakness in arm abduction and loss of sensation over the lateral shoulder. Examination reveals injury to the axillary nerve, which arises from the posterior cord of the brachial plexus and carries fibers from the C5 and C6 spinal levels. This nerve supplies a muscle that abducts, flexes, extends, and medially and laterally rotates the arm at the glenohumeral joint. In addition to arterial supply from branches of the thoracoacromial trunk, this muscle is also supplied by which artery that branches off the axillary artery and courses posteriorly to wrap around the surgical neck of the humerus?
Answer: Posterior circumflex humeral
The posterior circumflex humeral artery branches from the third part of the axillary artery and courses with the axillary nerve through the quadrangular space to supply the deltoid.
A 29-year-old long-distance cyclist reports numbness and tingling over the medial hand and weakness in finger abduction after a multi-day ride. The ulnar nerve (C8–T1), passes superficial to the flexor retinaculum and can become compressed in Guyon’s canal at the wrist in cyclists. In the forearm, it innervates the flexor carpi ulnaris and the medial half of the flexor digitorum profundus. Another nerve branches from the brachial plexus from the same cord as the ulnar nerve and gives cutaneous innervation to part of the upper limb distal to the humeroulnar joint. What is the name of this nerve?
Answer: Medial cutaneous nerve of the forearm
The ulnar nerve and medial cutaneous nerve of the forearm both originate from the medial cord (C8–T1); the latter provides sensory innervation to the skin of the anteromedial forearm.
A 55-year-old woman with poorly controlled primary hyperparathyroidism presents with bone pain and decreased bone density on DEXA scan. Excess parathyroid hormone (PTH) binds to receptors on osteoblasts, stimulating them to release RANK-L, which binds RANK on osteoclast precursors to increase bone resorption. This increased resorption primarily affects cortical bone, which contains embedded cells that coordinate remodeling activity (e.g. osteocytic osteolysis) and sense mechanical strain within the matrix. Through what microscopic structures do these embedded cells communicate with one another?
Answer: Canaliculi
Osteocytes embedded within the mineralized matrix communicate and exchange nutrients via slender cytoplasmic processes that extend through microscopic channels called canaliculi. Osteocytic osteolysis is a rapid, localized mineral release by osteocytes, whereas traditional bone remodeling involves coordinated resorption by osteoclasts and new bone formation by osteoblasts.
An infant with generalized hypotonia and poor neck muscle strength is noted to have a persistently straight cervical spine on lateral X-ray, indicating failure of the normal secondary curvature, the cervical lordosis, to form. During embryonic development, this same region shows a ventral neural bend (flexure) at the junction between the hindbrain and what structure?
Answer: The spinal cord
The cervical flexure forms between the hindbrain (rhombencephalon) and the spinal cord. Failure of normal postnatal muscle tone and head control prevents development of cervical lordosis, a normal secondary curvature alongside the lumbar lordosis.
A 64-year-old man is treated with topical 5-fluorouracil (5-FU) for a basal cell carcinoma arising from the stratum basale layer of the epidermis. This chemotherapeutic drug inhibits thymidylate synthase, preventing the methylation of dUMP to dTMP. By depleting thymidine nucleotides, 5-FU selectively targets malignant cells with high mitotic activity, leading to growth arrest and apoptosis. Another chemotherapeutic agent, methotrexate, indirectly inhibits the same pathway by depleting the cofactor required by thymidylate synthase. Which enzyme generates this cofactor?
Answer: Dihydrofolate reductase
Methotrexate inhibits dihydrofolate reductase, which regenerates methylene-THF, the cofactor thymidylate synthase needs to convert dUMP to dTMP.
A 58-year-old man develops weakness predominantly affecting his right leg. Imaging reveals a large left frontal lobe glioblastoma causing subfalcine herniation with medial displacement of the cingulate gyrus, compressing the artery known to be at risk for occlusion from this type of herniation. This focal deficit reflects the somatotopic organization of the primary motor cortex. Which artery on which side is most likely compressed in this condition?
Answer: Left anterior cerebral artery
A subfalcine herniation displaces the cingulate gyrus under the falx cerebri, compressing the ipsilateral anterior cerebral artery, which supplies the medial primary motor cortex controlling the contralateral leg.
A 36-year-old man undergoes surgical repair of a posterior humeral fracture. During the procedure, the surgeon identifies several anatomic spaces in the posterior shoulder region to locate and protect nearby neurovascular structures. The quadrangular space transmits the axillary nerve and posterior circumflex humeral artery, the triangular space transmits the circumflex scapular artery, and the triangular interval transmits one of the terminal branches of the brachial plexus and the deep brachial artery. The nerve that passes through the triangular interval is comprised of fibers from which spinal cord levels?
Answer: C5-T1
The radial nerve, a terminal branch of the posterior cord, passes through the triangular interval with the profunda brachii artery and contains fibers from C5 to T1.
A 35-year-old carpenter presents with difficulty extending his index finger after accidentally lacerating the dorsal aspect of his distal forearm, just proximal to the wrist. On examination, he can still extend his other fingers and wrist normally, but the index finger remains flexed when attempting to make a full hand extension. The nerve that innervates this muscle also innervates which muscle in the same compartment that forms a border of the anatomical snuff box and extends the tip of the thumb at the interphalangeal joint during a thumbs-up?
Answer: Extensor pollicis longus
The posterior interosseous nerve, a continuation of the deep branch of the radial nerve, innervates the extensor indicis and extensor pollicis longus, the latter forming part of the anatomical snuffbox and extending the thumb at the IP joint. The thumb does not have a PIP or a DIP since it has only two phalanges.
A 7-year-old boy with disproportionately short limbs but normal trunk length is diagnosed with achondroplasia involving constitutive activation of FGFR3. Microscopic analysis of his growth plate shows disorganization and premature closure of the epiphyseal cartilage, with failure of chondrocytes to proliferate and align into columns. Which zone of the growth plate would be most affected?
Answer: Zone of proliferation
In achondroplasia, excessive FGFR3 signaling suppresses chondrocyte proliferation, disrupting column formation in the proliferative zone of the growth plate and causing shortened long bones.
A woman taking isotretinoin for cystic acne becomes pregnant and delivers an infant with shortened limbs and facial malformations. The drug disrupts normal morphogen gradients that control limb bud patterning during weeks 5–8 of development. These gradients include SHH from the zone of polarizing activity, FGF from the apical ectodermal ridge, and retinoic acid from the proximal mesenchyme, which isotretinoin mimics and dysregulates. Retinoic acid also interacts with SHH signals from the prechordal plate to regulate activation of what family of genes that determine both the cranial-caudal segmentation of somites and the placement of limb buds?
Answer: Hox genes
HOX genes establish positional identity along the embryonic axis, directing where somites and limb buds form. Their expression is regulated by morphogen gradients such as retinoic acid and Sonic Hedgehog (SHH), which can be disrupted by teratogens like isotretinoin, leading to patterning defects.
You tell your kids to go outside and play with the neighbor’s kid. They return soon after, saying he’s weird and tried to eat his fingers and kept talking about passing kidney stones. Later, you notice the neighbor’s kid repeatedly banging his head against a tree. The deficient enzyme in this child normally combines PRPP with guanine to generate GMP or combines PRPP with what molecule to generate IMP?
Answer: Hypoxanthine
HGPRT normally salvages guanine to GMP and hypoxanthine to IMP, and deficiency causes Lesch-Nyhan syndrome with self-mutilation and uric acid overproduction.