Dermatome overlap and clinical implications US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Dermatome overlap and clinical implications. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Dermatome overlap and clinical implications US Medical PG Question 1: A 50-year-old male is brought to the dermatologist's office with complaints of a pigmented lesion. The lesion is uniformly dark with clean borders and no asymmetry and has been increasing in size over the past two weeks. He works in construction and spends large portions of his day outside. The dermatologist believes that this mole should be biopsied. To prepare the patient for the biopsy, the dermatologist injects a small amount of lidocaine into the skin around the lesion. Which of the following nerve functions would be the last to be blocked by the lidocaine?
- A. Pain
- B. Touch
- C. Temperature
- D. Sympathetic stimulation
- E. Pressure (Correct Answer)
Dermatome overlap and clinical implications Explanation: ***Pressure***
- **Pressure** sensation is mediated by **Aβ fibers**, which are relatively **larger** and **myelinated**, making them more resistant to local anesthetic blockade.
- Nerve fibers are blocked in a specific order, typically starting with smaller, unmyelinated fibers and ending with larger, myelinated fibers.
*Pain*
- **Pain** sensation is primarily carried by **unmyelinated C fibers** and **small myelinated Aδ fibers**, which are among the **first to be blocked** by local anesthetics.
- These fibers have a **high surface-to-volume ratio**, making them more susceptible to the action of lidocaine.
*Touch*
- **Touch** sensation is mediated by a mix of **Aβ and Aδ fibers**; light touch is typically blocked relatively early due to the involvement of smaller fibers.
- However, **crude touch** often persists longer than pain and temperature but is usually blocked before pressure.
*Temperature*
- **Temperature** sensation is primarily carried by **Aδ and C fibers**, making it one of the **earliest sensations to be blocked** by local anesthetic.
- These fibers are generally small and have high sensitivity to local anesthetic agents.
*Sympathetic stimulation*
- **Sympathetic nerve fibers** are typically **small, unmyelinated C fibers** and are generally the **first to be blocked** by local anesthetics.
- This early blockade can lead to **vasodilation** in the area due to the loss of sympathetic tone.
Dermatome overlap and clinical implications US Medical PG Question 2: A 65-year-old man presents to a clinic after 2 days of pain just below the right nipple. The pain radiates to the scapula. The rash was preceded by a burning and tingling pain in the affected region. His medical history is relevant for hypertension and hypercholesterolemia. He does not recall his vaccination status or childhood illnesses. A physical examination reveals stable vital signs and a vesicular rash distributed along the T4 dermatome. Which of the following is most appropriate for treating his condition and preventing further complications?
- A. Prednisone
- B. Valganciclovir
- C. Gabapentin
- D. Amitriptyline
- E. Famciclovir (Correct Answer)
Dermatome overlap and clinical implications Explanation: ***Famciclovir***
- This patient presents with classic **herpes zoster** (shingles): **prodrome of burning/tingling pain** followed by a **vesicular rash in a dermatomal distribution (T4)**.
- **Antiviral therapy** with famciclovir, valacyclovir, or acyclovir is the **primary treatment** for acute herpes zoster.
- Most effective when initiated **within 72 hours of rash onset** to reduce duration of pain, accelerate rash healing, and **decrease risk of postherpetic neuralgia (PHN)**.
- Famciclovir is a **prodrug of penciclovir** with excellent oral bioavailability.
*Prednisone*
- Corticosteroids are **not the primary treatment** for acute herpes zoster and do not prevent viral replication.
- Evidence for corticosteroids reducing **postherpetic neuralgia** is limited and controversial.
- May be used as **adjunctive therapy** in select cases for severe inflammation, but antivirals remain first-line.
*Valganciclovir*
- Valganciclovir is specific for **cytomegalovirus (CMV)** infections, not **varicella-zoster virus (VZV)**.
- While structurally related to other antivirals, it has **poor activity against VZV** compared to famciclovir, valacyclovir, or acyclovir.
- Used primarily in immunocompromised patients with CMV retinitis or organ transplant recipients.
*Gabapentin*
- Gabapentin is an **antiepileptic/neuropathic pain agent** used to treat **postherpetic neuralgia (PHN)** after it develops.
- Does **not treat the acute viral infection** or prevent PHN when started during acute phase.
- Started if chronic neuropathic pain persists **>90 days** after rash onset.
*Amitriptyline*
- Amitriptyline is a **tricyclic antidepressant (TCA)** effective for managing chronic **neuropathic pain** including PHN.
- Like gabapentin, it treats the **chronic pain complication**, not the acute viral infection.
- Does not prevent PHN development when used during acute shingles phase.
Dermatome overlap and clinical implications US Medical PG Question 3: A 63-year-old man comes to the physician because of a 2-day history of a painful rash on his right flank. Two years ago, he underwent cadaveric renal transplantation. Current medications include tacrolimus, mycophenolate mofetil, and prednisone. Examination shows an erythematous rash with grouped vesicles in a band-like distribution over the patient's right flank. This patient is at greatest risk for which of the following complications?
- A. Loss of vision
- B. Temporal lobe inflammation
- C. Postherpetic neuralgia (Correct Answer)
- D. Sensory neuropathy
- E. Urinary retention
Dermatome overlap and clinical implications Explanation: ***Postherpetic neuralgia***
- This patient has **herpes zoster (shingles)**, characterized by a painful vesicular rash in a dermatomal distribution. **Postherpetic neuralgia (PHN)** is the most common complication of herpes zoster, defined as persistent neuropathic pain lasting more than 90 days after rash onset.
- This patient has **multiple risk factors for PHN**: advanced age (63 years), immunocompromised state (renal transplant on tacrolimus, mycophenolate, and prednisone), and severe acute pain. PHN occurs in **10-18% of all herpes zoster cases** but can affect **50-70% of immunocompromised patients**.
- PHN results from nerve damage caused by VZV reactivation and inflammation, leading to chronic neuropathic pain that can persist for months to years after the rash resolves.
*Loss of vision*
- Loss of vision occurs with **herpes zoster ophthalmicus**, which involves the ophthalmic division of the trigeminal nerve (V1) and can lead to keratitis, uveitis, and vision loss.
- This patient's rash is on the **right flank** (thoracic/lumbar dermatome), not the ophthalmic distribution, making vision loss unlikely.
*Temporal lobe inflammation*
- **Temporal lobe encephalitis** is a rare complication of disseminated VZV infection, typically occurring in severely immunocompromised patients (AIDS, chemotherapy).
- While this transplant patient is immunosuppressed, localized dermatomal zoster rarely causes CNS complications. Encephalitis would present with altered mental status, seizures, and focal neurological deficits.
*Urinary retention*
- Urinary retention can occur when herpes zoster affects the **sacral dermatomes (S2-S4)**, involving autonomic nerves that innervate the bladder.
- This patient's rash is on the **right flank** (thoracic or lumbar dermatome), making sacral involvement and urinary retention unlikely.
*Sensory neuropathy*
- While herpes zoster does cause sensory nerve damage, "sensory neuropathy" is not a standard clinical term for herpes zoster complications. The specific consequence of this nerve damage that persists as a complication is **postherpetic neuralgia** (PHN).
- All nerve damage from zoster is essentially sensory in nature, but when asking about the greatest risk for complications, **postherpetic neuralgia** is the recognized and most common complication, especially in elderly immunocompromised patients.
Dermatome overlap and clinical implications US Medical PG Question 4: A 23-year-old man presents to the emergency department with a severe headache. The patient states he gets sudden, severe pain over his face whenever anything touches it, including shaving or putting lotion on his skin. He describes the pain as electric and states it is only exacerbated by touch. He is currently pain free. His temperature is 98.1°F (36.7°C), blood pressure is 127/81 mmHg, pulse is 87/min, respirations are 15/min, and oxygen saturation is 98% on room air. Neurological exam is within normal limits, except severe pain is elicited with light palpation of the patient’s face. The patient is requesting morphine for his pain. Which of the following is the most likely diagnosis?
- A. Tension headache
- B. Migraine headache
- C. Cluster headache
- D. Trigeminal neuralgia (Correct Answer)
- E. Malingering
Dermatome overlap and clinical implications Explanation: ***Trigeminal neuralgia***
- This patient's symptoms of **sudden, severe, electric-shock-like pain** in the face, triggered by light touch (e.g., shaving, lotion), are classic for **trigeminal neuralgia**.
- The pain is typically unilateral, short-lasting, and occurs in the distribution of one or more branches of the **trigeminal nerve**.
*Tension headache*
- Tension headaches typically present as a **constant, dull aching** or pressure sensation, often described as a band around the head.
- They are usually not associated with electric shock-like pain or specific triggers like light touch to the face.
*Migraine headache*
- Migraines are characterized by **throbbing, unilateral pain** often accompanied by **nausea, photophobia, and phonophobia**.
- While severe, they do not typically present with the lancinating, trigger-point-induced pain pattern seen in this patient.
*Cluster headache*
- Cluster headaches are characterized by **severe, unilateral pain**, often orbital or periorbital, accompanied by **autonomic symptoms** (e.g., lacrimation, rhinorrhea, ptosis) on the affected side.
- The pain is usually constant during an attack and is not described as electric shock-like or triggered by light touch, unlike trigeminal neuralgia.
*Malingering*
- While the patient is requesting morphine, his description of pain and its specific triggers are highly consistent with a recognized neurological condition, **trigeminal neuralgia**.
- **Malingering** involves deliberately fabricating or exaggerating symptoms for external incentives, which is not supported by the classic presentation of a distinct medical condition.
Dermatome overlap and clinical implications US Medical PG Question 5: A 37-year-old man presents to his primary care provider complaining of bilateral arm numbness. He was involved in a motor vehicle accident 3 months ago. His past medical history is notable for obesity and psoriatic arthritis. He takes adalimumab. His temperature is 99.3°F (37.4°C), blood pressure is 130/85 mmHg, pulse is 90/min, and respirations are 18/min. On exam, superficial skin ulcerations are found on his fingers bilaterally. His strength is 5/5 bilaterally in shoulder abduction, arm flexion, arm extension, wrist extension, finger abduction, and thumb flexion. He demonstrates loss of light touch and pinprick response in the distal tips of his 2nd and 5th fingertips and over the first dorsal web space. Vibratory sense is intact in the bilateral upper and lower extremities. Which of the following nervous system structures is most likely affected in this patient?
- A. Cuneate fasciculus
- B. Ventral horns
- C. Anterior corticospinal tract
- D. Spinocerebellar tract
- E. Ventral white commissure (Correct Answer)
Dermatome overlap and clinical implications Explanation: ***Ventral white commissure***
- The patient presents with **bilateral loss of pain (pinprick) and light touch sensation** in the upper extremity fingertips, while **vibratory sense is intact** and **motor strength is fully preserved (5/5)**. This dissociated sensory loss pattern is pathognomonic for a lesion affecting the **ventral white commissure**.
- The ventral white commissure contains **decussating fibers of the spinothalamic tract**, which carry pain and temperature sensation from the contralateral body. A lesion here (classically seen in **syringomyelia** affecting the cervical spinal cord) causes **bilateral loss of pain and temperature sensation** in a characteristic distribution while **sparing the dorsal columns** (vibratory sense and proprioception remain intact) and motor pathways.
- The **superficial skin ulcerations** on his fingers are explained by chronic loss of protective pain sensation, leading to unnoticed repetitive trauma. The motor vehicle accident 3 months ago may have precipitated or worsened an underlying syrinx.
- This is the classic **"cape-like" or suspended sensory loss** pattern, though it can present with focal dermatomal involvement as in this case.
*Cuneate fasciculus*
- The cuneate fasciculus is part of the **dorsal column-medial lemniscal pathway** that carries **vibratory sense, proprioception, and fine discriminative touch** from the upper extremities.
- A lesion here would cause **loss of vibratory sense** and proprioception, which are explicitly **intact** in this patient, making this option incorrect.
*Ventral horns*
- The ventral horns contain **lower motor neuron cell bodies** that innervate skeletal muscles.
- Damage would cause **motor deficits** including weakness (reduced strength), muscle atrophy, and fasciculations, none of which are present in this patient who has normal 5/5 strength throughout.
*Anterior corticospinal tract*
- This tract mediates **voluntary motor control**, primarily of axial and proximal muscles.
- Lesions would result in **motor weakness or spasticity**, not the isolated sensory deficits seen in this patient.
*Spinocerebellar tract*
- The spinocerebellar tracts carry **unconscious proprioceptive information** to the cerebellum for motor coordination.
- Damage would manifest as **ataxia, dysmetria, and incoordination**, which are not described in this patient's presentation.
Dermatome overlap and clinical implications US Medical PG Question 6: A 59-year-old woman presents to the emergency room with severe low back pain. She reports pain radiating down her left leg into her left foot. She also reports intermittent severe lower back spasms. The pain started after lifting multiple heavy boxes at her work as a grocery store clerk. She denies bowel or bladder dysfunction. Her past medical history is notable for osteoporosis and endometrial cancer. She underwent a hysterectomy 20 years earlier. She takes alendronate. Her temperature is 99°F (37.2°C), blood pressure is 135/85 mmHg, pulse is 85/min, and respirations are 22/min. Her BMI is 21 kg/m^2. On exam, she is unable to bend over due to pain. Her movements are slowed to prevent exacerbating her muscle spasms. A straight leg raise elicits severe radiating pain into her left lower extremity. The patient reports that the pain is worst along the posterior thigh and posterolateral leg into the fourth and fifth toes. Palpation along the lumbar vertebral spines demonstrates mild tenderness. Patellar reflexes are 2+ bilaterally. The Achilles reflex is decreased on the left. Which nerve root is most likely affected in this patient?
- A. L5
- B. S2
- C. L3
- D. L4
- E. S1 (Correct Answer)
Dermatome overlap and clinical implications Explanation: ***S1***
- Pain radiating to the **posterior thigh**, **posterolateral leg**, and into the **fourth and fifth toes** is characteristic of **S1 dermatome involvement**.
- A **decreased Achilles reflex** (ankle jerk reflex) specifically points to compromise of the **S1 nerve root**.
*L5*
- **L5 radiculopathy** typically causes pain and sensory deficits in the **dorsum of the foot** and into the **first, second, and third toes**.
- Motor weakness often affects **foot dorsiflexion** and **toe extension**, not primarily the Achilles reflex.
*S2*
- **S2 radiculopathy** would primarily affect sensation along the **posterior thigh** and **calf**, with possible involvement of the **plantar aspect of the foot**.
- It does not typically cause a decrease in the **Achilles reflex**, which is predominantly S1.
*L3*
- **L3 radiculopathy** typically presents with pain and sensory changes along the **anterior thigh** and possibly the **medial knee**.
- It can affect the **patellar reflex**, which is intact in this patient, and does not cause pain in the posterior leg or foot.
*L4*
- **L4 radiculopathy** typically causes pain and sensory changes over the **anterior thigh**, **medial leg**, and potentially the **medial malleolus**.
- It often presents with weakness in **quadriceps muscle** and can cause a diminished **patellar reflex**, which is normal in this patient.
Dermatome overlap and clinical implications US Medical PG Question 7: A 57-year-old man presents to the ED complaining of back and left leg pain. He was lifting heavy furniture while helping his daughter move into college when all of sudden he felt a sharp pain at his back. The pain is described as severe, worse with movement, and shoots down his lateral thigh. The patient denies any bowel/urinary incontinence, saddle anesthesia, weight loss, or weakness. He denies any past medical history but endorses a family history of osteoporosis. He has been smoking 1 pack per day for the past 20 years. Physical examination demonstrated decreased sensation at the left knee, decreased patellar reflex, and a positive straight leg test. There is diffuse tenderness to palpation at the lower back but no vertebral step-offs were detected. What is the most likely etiology for this patient’s pain?
- A. Vertebral compression fracture
- B. Disc herniation at the L4/L5 vertebra
- C. Spinal metastasis from lung cancer
- D. Disc herniation at the L3/L4 vertebra (Correct Answer)
- E. Lumbar muscle sprain
Dermatome overlap and clinical implications Explanation: ***Disc herniation at the L3/L4 vertebra***
- The patient's symptoms of **acute back pain radiating down the lateral thigh** after lifting, combined with **decreased sensation at the left knee** and a **decreased patellar reflex**, are classic signs of L3/L4 nerve root compression.
- A **positive straight leg test** also supports nerve root irritation, and the absence of red flag symptoms like incontinence or saddle anesthesia makes a simple disc herniation more likely than other serious conditions.
*Vertebral compression fracture*
- While lifting heavy objects can cause compression fractures, these usually present with more **severe, localized pain** that is not typically radiating with specific dermatomal or reflex changes.
- Absence of **vertebral step-offs** or significant predisposing factors for a fracture (e.g., severe osteoporosis, trauma) makes this less likely given the specific neurological findings.
*Disc herniation at the L4/L5 vertebra*
- An L4/L5 disc herniation would typically cause symptoms related to the **L5 nerve root**, such as pain radiating down the **lateral leg into the foot**, **weakness in dorsiflexion of the ankle** or **big toe**, and potentially a **decreased medial hamstring reflex**.
- The patient's reported symptoms (lateral thigh pain, decreased knee sensation, decreased patellar reflex) are more consistent with **L4 nerve root** involvement.
*Spinal metastasis from lung cancer*
- Although the patient has a **smoking history** and could be at risk for lung cancer, this diagnosis typically presents with more **insidious onset** of unexplained back pain, often with **weight loss**, and sometimes with more profound neurological deficits or bone pain not relieved by rest.
- The acute onset after an inciting event and specific neurological findings of a single nerve root are less suggestive of metastasis.
*Lumbar muscle sprain*
- A muscle sprain would typically present with **localized back pain**, often worsened by movement, but would **not involve radicular pain** shooting down the leg, nor would it cause specific **neurological deficits** like decreased sensation or reflex changes.
- The positive straight leg test and neurological findings rule out a simple muscle sprain.
Dermatome overlap and clinical implications US Medical PG Question 8: A 54-year-old woman comes to the physician because of a 1-day history of fever, chills, and double vision. She also has a 2-week history of headache and foul-smelling nasal discharge. Her temperature is 39.4°C (103°F). Examination shows mild swelling around the left eye. Her left eye does not move past midline on far left gaze but moves normally when looking to the right. Without treatment, which of the following findings is most likely to occur in this patient?
- A. Hemifacial anhidrosis
- B. Jaw deviation
- C. Absent corneal reflex (Correct Answer)
- D. Relative afferent pupillary defect
- E. Hypoesthesia of the earlobe
Dermatome overlap and clinical implications Explanation: ***Absent corneal reflex***
- This patient's symptoms (fever, chills, headache, foul-smelling nasal discharge, periorbital swelling, and ophthalmoplegia) suggest **cavernous sinus thrombosis** secondary to a sinus infection.
- The cavernous sinus contains cranial nerves III, IV, VI, V1, and V2. Untreated, the infection and thrombosis can easily spread to affect **cranial nerve V1 (ophthalmic branch of trigeminal nerve)**, leading to an absent corneal reflex.
*Hemifacial anhidrosis*
- This symptom, along with ptosis and miosis, is indicative of **Horner's syndrome**, which results from damage to the ipsilateral **sympathetic pathway**.
- While cavernous sinus thrombosis can rarely involve sympathetic fibers, it's not the most direct or common neurological sequela compared to trigeminal nerve involvement.
*Jaw deviation*
- **Jaw deviation** typically occurs due to weakness or paralysis of the **motor branch of the trigeminal nerve (V3)**, which innervates the muscles of mastication.
- Cavernous sinus thrombosis primarily affects V1 and V2, and V3 involvement, while possible, is less common and usually presents later than V1 or V2 deficits.
*Relative afferent pupillary defect*
- A relative afferent pupillary defect (RAPD, or Marcus Gunn pupil) indicates a lesion in the **afferent visual pathway** (e.g., optic nerve or retina).
- While vision can be affected in cavernous sinus thrombosis due to optic nerve compression or venous congestion, RAPD is not the most direct or specific neurological complication expected from the provided symptoms.
*Hypoesthesia of the earlobe*
- Sensation to the earlobe is primarily supplied by the **great auricular nerve (C2-C3 cervical spinal nerves)** with minor contribution from the **auricular branch of the vagus nerve (CN X)**.
- Cavernous sinus thrombosis does not involve these nerves, and hypoesthesia of the earlobe is not a characteristic finding.
Dermatome overlap and clinical implications US Medical PG Question 9: A 28-year-old man comes to the physician because of a 3-month history of pain in his left shoulder. He is physically active and plays baseball twice a week. The pain is reproduced when the shoulder is externally rotated against resistance. Injury of which of the following tendons is most likely in this patient?
- A. Infraspinatus (Correct Answer)
- B. Subscapularis
- C. Pectoralis major
- D. Supraspinatus
- E. Teres major
Dermatome overlap and clinical implications Explanation: ***Infraspinatus***
- Pain during **external rotation against resistance** is a classic sign of infraspinatus tendon injury, as it is a primary muscle for this action.
- The patient's history of playing baseball and experiencing pain, especially with resistive external rotation, points to an injury of this **rotator cuff muscle**.
*Subscapularis*
- The subscapularis primarily causes **internal rotation** of the shoulder; injury would typically present with pain during resisted internal rotation, not external.
- While it is a rotator cuff muscle, its function does not align with the specific maneuver causing pain described in the patient.
*Pectoralis major*
- The pectoralis major is a large chest muscle involved primarily in **adduction**, **internal rotation**, and **flexion of the humerus**, not external rotation.
- Injury to this muscle would present with pain during these specific movements, not resisted external rotation.
*Supraspinatus*
- The supraspinatus is primarily involved in **initiation of abduction** and helps stabilize the shoulder joint, and pain would usually be elicited during these movements.
- While a common site of rotator cuff injury, its function does not directly cause pain with resisted external rotation as described.
*Teres major*
- The teres major acts as an **adductor** and **internal rotator** of the humerus, similar to the latissimus dorsi.
- Pain from a teres major injury would be associated with these actions, not with resisted external rotation.
Dermatome overlap and clinical implications US Medical PG Question 10: A 58-year-old diabetic man with multiple thoracic vertebral compression fractures presents with progressive lower extremity dysfunction. He has spastic paraparesis with hyperreflexia, bilateral Babinski signs, and a sensory level at T10. However, he also has areflexic bladder, saddle anesthesia, and absent bulbocavernosus reflex. Upper extremities are completely normal. Synthesize the anatomical explanation for this mixed upper and lower motor neuron presentation.
- A. Complete spinal cord transection at T10 level
- B. Conus medullaris syndrome with additional cord compression (Correct Answer)
- C. Cauda equina syndrome with coincidental cord pathology
- D. Combined central cord and conus medullaris injury
- E. Diabetic lumbosacral radiculoplexus neuropathy with cord compression
Dermatome overlap and clinical implications Explanation: ***Conus medullaris syndrome with additional cord compression***
- The **conus medullaris** represents the terminal end of the spinal cord (L1-L2), where damage causes a mix of **UMN signs** (from the cord itself) and **LMN signs** (from exiting sacral roots).
- This patient exhibits **UMN signs** (spasticity, hyperreflexia, Babinski) from cord compression at T10, alongside **LMN signs** (areflexic bladder, saddle anesthesia, absent bulbocavernosus reflex) specific to the **conus/sacral segments**.
*Complete spinal cord transection at T10 level*
- A complete transection at T10 would cause **pure UMN signs** below the level of the lesion and would not selectively cause isolated **areflexic bladder** or **saddle anesthesia** typical of conus/root injury.
- It would typically result in a **spastic bladder** over time once the period of spinal shock has resolved, unlike this patient's LMN presentation.
*Cauda equina syndrome with coincidental cord pathology*
- **Cauda equina syndrome** involves compression of lumbosacral nerve roots and results in a **pure LMN lesion** without Babinski signs or hyperreflexia.
- While cord pathology could explain UMN signs, the specific clustering of sacral symptoms with thoracic compression points more directly to the **conus medullaris** transition zone.
*Combined central cord and conus medullaris injury*
- **Central cord syndrome** almost exclusively affects the **cervical spine**, leading to upper extremity weakness greater than lower extremity weakness.
- Since the patient's **upper extremities** are normal, a central cord mechanism is anatomically inconsistent with this presentation.
*Diabetic lumbosacral radiculoplexus neuropathy with cord compression*
- **Diabetic radiculoplexus neuropathy** (amyotrophy) typically presents with **acute, asymmetric thigh pain** and proximal muscle wasting, which is not described here.
- While diabetes is a risk factor for neuropathies, it does not explain the anatomical clustering of **saddle anesthesia** and **absent bulbocavernosus reflex** as precisely as a conus lesion.
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