An otherwise healthy 45-year-old man comes to the physician because of a painful ulcer on his tongue for 3 days. Examination shows a shallow, tender 5-mm wide ulcer on the lateral aspect of the tongue, adjacent to his left first molar. There is no induration surrounding the ulcer or cervical lymphadenopathy. A lesion of the cranial nerve responsible for the transmission of pain from this ulcer would most likely result in which of the following?

Loss of sensation in the anterior two-thirds of the tongue

Loss of taste from the supraglottic region

Lateral deviation of the tongue

Inability to wrinkle the forehead

Decreased sensation in the upper lip

A 54-year-old man is brought to the emergency department 30 minutes after being hit by a car while crossing the street. He had a left-sided tonic-clonic seizure and one episode of vomiting while being transported to the hospital. On arrival, he is not oriented to person, place, or time. Physical examination shows flaccid paralysis of all extremities. A CT scan of the head is shown. This patient's symptoms are most likely the result of a hemorrhage in which of the following structures?

Between the arachnoid mater and the pia mater

Between the dura mater and the arachnoid mater

Between the skull and the dura mater

A 55-year-old woman with a 1-year history of left-sided tinnitus is diagnosed with a tumor at the left cerebellopontine angle affecting the glossopharyngeal nerve. Sialometry shows decreased production of saliva from the left parotid gland. The finding on sialometry is best explained by a lesion of the nerve that is also responsible for which of the following?

Afferent limb of the gag reflex

Afferent limb of the cough reflex

Taste sensation of tip of the tongue

Cranial nerves and their pathways for USMLE Step 3: High-Yield Anatomy Lessons

Cranial Nerves Overview - The Cranial Crew

Cranial Nerves and Foramina Anatomical Chart

📌 Name Mnemonic: 'On Old Olympus' Towering Top, A Finn And German Viewed Some Hops' 📌 Function Mnemonic: 'Some Say Marry Money, But My Brother Says Big Brains Matter More'

#	Name	Exit Foramen	Function
I	Olfactory	Cribriform Plate	Sensory
II	Optic	Optic Canal	Sensory
III	Oculomotor	Superior Orbital Fissure	Motor
IV	Trochlear	Superior Orbital Fissure	Motor
V	Trigeminal	SOF, Foramen Rotundum, Ovale	Both
VI	Abducens	Superior Orbital Fissure	Motor
VII	Facial	Internal Auditory Meatus	Both
VIII	Vestibulocochlear	Internal Auditory Meatus	Sensory
IX	Glossopharyngeal	Jugular Foramen	Both
X	Vagus	Jugular Foramen	Both
XI	Accessory	Jugular Foramen	Motor
XII	Hypoglossal	Hypoglossal Canal	Motor

Sensory Nerves (I, II, VIII) - Just The Inputs

CN I (Olfactory): Purely sensory for smell. The only cranial nerve to directly enter the cerebrum, bypassing the thalamus.
CN II (Optic): Purely sensory for vision. Axons of retinal ganglion cells.
CN VIII (Vestibulocochlear): Purely sensory for hearing and balance.

Cranial Nerves: Bottom View of Brain

⭐ Anosmia (loss of smell) with CSF rhinorrhea is a classic sign of a cribriform plate fracture, often from head trauma.

Motor Nerves (III, IV, VI, XI, XII) - The Action Heroes

📌 Mnemonic (Extraocular): $LR_6(SO_4)R_3$

Oculomotor (CN III), Trochlear (CN IV), Abducens (CN VI):
- Control all extraocular muscles for precise eye movement.
- CN VI: Lateral Rectus (abducts eye).
- CN IV: Superior Oblique (depresses & intorts).
- CN III: All other recti & inferior oblique.
Accessory (CN XI): Innervates Sternocleidomastoid & Trapezius (head turn, shoulder shrug).
Hypoglossal (CN XII): Controls tongue musculature.

Extraocular muscles and cranial nerve innervation

⭐ CN XII Lesion: Tongue deviates toward the side of the lesion ("lick your wounds").

Mixed Nerves (V, VII, IX, X) - Doing It All

These nerves carry a combination of motor, sensory, and parasympathetic fibers, handling complex functions.

Nerve	Branchial Motor	Visceral Motor (Parasympathetic)	General Sensory	Special Sensory (Taste)
V	Mastication	-	Face, sinuses, cornea	-
VII	Facial expression	Lacrimal, submandibular, sublingual glands	-	Anterior 2/3 of tongue
IX	Stylopharyngeus	Parotid gland	Posterior 1/3 tongue, carotid body/sinus	Posterior 1/3 of tongue
X	Pharynx, larynx	Thoraco-abdominal viscera	Aortic arch chemo/baroreceptors	Epiglottis

⭐ The gag reflex tests two nerves: the afferent limb is CN IX (Glossopharyngeal), sensing the touch, while the efferent limb is CN X (Vagus), causing pharyngeal contraction.

Brainstem Nuclei & Lesions - The Control Center

Rule of 4s: A key to localizing brainstem lesions.
- 4 Medial Structures (all start with M): Motor pathway, Medial lemniscus, MLF, Motor nuclei (CN 3, 4, 6, 12).
- 4 Lateral Structures (all start with S): Spinocerebellar, Spinothalamic, Sensory nucleus (CN5), Sympathetic pathway.
- 4 CNs each in: Medulla (9, 10, 11, 12), Pons (5, 6, 7, 8), and above (1, 2, 3, 4).

Rule of 4s for Brainstem Lesions & Cranial Nerve Deficits

⭐ Lateral Medullary (Wallenberg) Syndrome from PICA occlusion is the most common brainstem stroke.

High‑Yield Points - ⚡ Biggest Takeaways

CN III palsy presents with a "down and out" gaze, ptosis, and mydriasis due to unopposed CN IV and VI action.

Bell's palsy (CN VII) causes ipsilateral upper and lower facial paralysis, distinguishing it from a central (UMN) lesion.

The cavernous sinus houses CN III, IV, V1, V2, and VI; thrombosis can cause multiple ophthalmoplegias.

Jugular foramen syndrome involves dysfunction of CN IX, X, and XI.

The gag reflex is mediated by CN IX (afferent) and CN X (efferent).

The corneal reflex involves CN V1 (afferent) and CN VII (efferent).

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