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USMLE STEP1 Exam (page: 10)
USMLE Step1
Updated on: 15-Feb-2026

Viewing Page 10 of 171
STEP1 PDF 845 Questions

Which label in following figure indicates the typical plane of separation at which retinal detachment occurs?

  1. A
  2. B
  3. C
  4. D
  5. E

Answer(s): D

Explanation:

Retinal separation typically occurs at the interface between the retinal pigment epithelium and the outer limit of the sensory (neural) retina. The weakness of this plane is attributed to the manner in which the retina develops, a process that involves obliteration of the space between two of the layers of the optic cup--an inner layer from which the sensory retina arises and an outer layer from which the retinal pigment epithelium arises. Other retinal layers are bridged by neuronal processes, and Müller cells (the retina's glial cells) span the entire thickness of the neural retina. Plane A(choice A) marks the boundary between the nerve fiber layer above and the ganglion cell layer below. The nerve fiber layer is composed of axons of the retinal ganglion cells. Plane B (choice B) is within the inner plexiform layer, the site of synaptic contacts between bipolar neurons, retinal ganglion cells, and amacrine cells. Plane C (choice C) is within the outer plexiform layer, the site of synapses among bipolar cells, rods and cones, and horizontal cells. The boundary between the choroid (of the middle vascular tunic or uvea) and the sclera (of the external, fibrous tunic) is marked by plane E (choice E).



A 28-year-old male patient suffering from head trauma resulting from a car accident is brought unconscious to the emergency room. In performing the pupillary light reflex, you notice that even though the left pupil constricts when you shine a light directly into the left eye, it does not do so when you shine a light into the right eye. This is best explained by a disconnection between which of the following bilateral structures?

  1. Edinger-Westphal nucleus
  2. habenula
  3. inferior colliculus
  4. lateral geniculate nucleus
  5. medial geniculate nucleus

Answer(s): A

Explanation:

The central visual pathway for the papillary light reflex is organized as follows: fibers from the ganglionic layer of the retina project posteriorly to the pretectum, which in turn innervates the Edinger- Westphal nucleus. Preganglionic parasympathetic neurons in the Edinger- Westphal nucleus project to the ciliary ganglion, which sends postganglionic parasympathetic innervation back to the constrictor pupillae of the eye. The Edinger-Westphal nuclei from each side of the midbrain are also connected to each other by projections running through the posterior commissure. Disconnection of these fibers will result in loss of the consensual papillary light reflex on the contralateral side, as happens in this case. The habenula (choice B) is a nucleus of the thalamus, which does not participate in the central visual pathways. The lateral geniculate nucleus (choice D) receives fibers from the ganglionic layer of the retina. However, fibers participating in the papillary light reflex run through this structure without synapsing and terminate in the pretectum. Thus, the lateral geniculate nucleus does not participate in the pupillary light reflex. The inferior colliculus (choice C) and the medial geniculate nucleus (choice E) are components of the auditory system.



A 45-year-old female patient presents to the emergency room with a headache and complains of abnormal sensations on the left side of her body. She claims that the sensory changes came on rapidly in the last few hours. Her laboratory reports come back normal, a spinal tap reveals normal cerebrospinal fluid (CSF), and her mental ability seems good. You perform a neurological examination and find she has greatly reduced sensation on the entire left side of her body, including her face. The sensory loss includes all modalities. The motor examination is normal, as is her visual examination and hearing examination. Based on the neurological findings, which of the following thalamic nuclei would be involved?

  1. lateral and medial geniculate nuclei
  2. lateral dorsal and lateral posterior nuclei
  3. pulvinar
  4. ventral anterior and ventral lateral nuclei
  5. ventral posterior lateral and ventral posterior medial nuclei

Answer(s): E

Explanation:

The ventral posterior lateral (VPL) and ventral posterior medial (VPM) nuclei are sensory relay thalamic nuclei. The lateral and medial geniculate nuclei (choice A) are respectively concerned with visual and auditory sensory information. The lateral dorsal (LD) and lateral posterior (LP) nuclei (choice B), and the pulvinar (choice C) are association or multimodal relay nuclei. The ventral anterior (VA) and ventral lateral (VL) are motor relay nuclei.



A patient has been admitted for hematemesis (vomiting of blood). Endoscopic examination reveals bleeding esophageal varices resulting from portal obstruction. These varices represent anastomoses between branches of which of the following?

  1. inferior vena cava with a patent ductus venosus
  2. left gastric, azygos, and hemiazygos veins
  3. right gastric vein and the inferior vena cava
  4. superior, middle, and inferior rectal veins
  5. veins running on the ligamentum teres and the epigastric veins

Answer(s): B

Explanation:

Obstruction of the portal vein results in an increase in the collateral circulation between veins that normally drain to the portal vein and those that drain to the systemic veins. Choices A, B, D, and E all represent possible collateral venous circulation in case of portal obstruction. Choice A is rare because the ductus venosus closes after birth. Choice B is correct because varicose veins in this region give rise to esophageal varices. Choice D results in varicose veins in the rectal region. Choice C is incorrect because there is no connection between the right gastric vein and the inferior vena cava. In choice E, enlargement of the epigastric veins results in varicose veins radiating from the umbilicus, the caput medusae.



A 15-year-old high school football player is brought to the emergency room. On examination, his right lower limb is deformed and swollen around the knee. At full extension, there is valgus instability, suggestive of knee dislocation. The patient is in great pain and there is a concern for concomitant vascular and nervous injuries. Palpation of the dorsalis pedis artery reveals a normal pulse. However, neurological examination reveals impaired dorsiflexion of the foot with decreased sensation in the space between digits 1 and 2.
Which of the following nerves is affected?

  1. deep peroneal (fibular) nerve
  2. femoral nerve
  3. saphenous nerve
  4. superficial peroneal (fibular) nerve
  5. tibial nerve

Answer(s): A

Explanation:

The deep peroneal (fibular) nerve innervates the muscles responsible for dorsiflexion in the anterior compartment of the leg. It also provides for the cutaneous innervation of the space between digits 1 and 2. The femoral nerve (choice B) innervates muscles in the anterior compartment of the thigh and the skin of the medial aspect of the leg by a continuing branch, the saphenous nerve (choice C). The superficial peroneal (fibular) nerve (choice D) innervates the lateral compartment of the leg and muscles responsible for foot eversion. The tibial nerve (choice E) innervates the posterior compartment of the leg and the muscles responsible for plantar flexion.



Viewing Page 10 of 171



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