Hydrophilic (polar) molecules are able to diffuse across the olfactory mucosa, while hydrophobic (nonpolar) molecules are first bound to transport proteins. We are looking for the answer choice that contains one polar and one nonpolar molecule. Ethanol is polar and methane is nonpolar, making this the correct answer choice.

Methane, ethane, and benzene are all nonpolar. Ethanol and ammonia are both polar.

Olfactory neurons are the sole nerve cells that receive sensory information from the external environment. Basal cells will differentiate into neurons while columnar cells secrete mucosa. The true statement is that cilia density on the ends of the olfactory neurons determines odor sensitivity; the cilia contain many odor-specific receptor proteins.

After taste receptor cells contact solutes in the saliva, action potentials cause a calcium influx through ion channels, which leads to the release of the neurotransmitter that induces an action potential in the afferent nerve. This action potential is carried to the brain for integration and interpretation.

Chloride ions do not enter the receptor during this process.

The incus, along with the malleus and stapes, are three bones found in the middle ear, which act as a lever system to transmit sound waves to the oval window.

The pinna is simply the skin and cartilage component of the outer ear. The semicircular canals are found in the inner ear, and are responsible for adapting the head and body to positional changes, thus maintaining balance. The perilymph is the fluid found within the inner ear. 

The malleus, incus, and stapes are the three bones of the ear. Sound enters the external ear by using air as a medium. In the middle ear, vibrations from the air transition to vibrations through bone. The inner ear converts these bone vibrations to fluid vibrations in the cochlea, which converts the fluid vibrations to electrical stimuli for the nervous system.

The vestibular apparatus is the name given for the entire inner ear structure that interprets position. It contains substructures such as the semicircular canals, the ampulla, and the crista. The crista are the specialized hair cells that respond to fluid movement; fluid does not move within the crista themselves.

All other listed answers are true.

The cochlear and vestibular nerves join to form the auditory nerve. The crista are specialized hair cells that help in postural equilibrium and send information via the vestibular nerve. The incus is one of the three auditory bones (the others include the malleus and the stapes), the motion of which is part of sound reception. This information is transmitted via the cochlear nerve.  Finally, the cochlea is the fluid-filled structure of the inner ear that translates movement into vibrations (also involved in sound reception).  All of the given structures take part in transmitting information to the acoustic nerve.

The first structure that light will encounter as it enters the eye is the cornea, a transparent protein disc designed to refract light toward the eye interior. The aqueous humor is the liquid medium between the cornea and the lens of the eye. Light will pass through the aqueous humor and the pupil before hitting the lens. The lens then focuses the light onto the back of the eye. The vitreous humor is the liquid medium between the lens and the retina at the back of the eye.

The iris is used to control the shape of the pupil, and does not contain photoreceptors. All photoreceptors are located on the retina, which lines the back wall of the eye. A small region known as the fovea centralis houses a large number of cones (photoreceptors) at the back of the eye, but is part of the greater retinal structure.

The only true statement given is that light will pass through the aqueous humor before passing through the vitreous humor.

Afferent pathway, containing nerve axons from both eyes. Again, this question requires us to draw on our knowledge of the nervous system, and in particular, the eye. One must remember that the information leading towards the brain is found in the afferent nerves. Information leading away from the central nervous system is found in the efferent nerves. Therefore, answer choices, ‘efferent pathway, containing nerve axons from only one eye’, and ‘efferent pathway, containing nerve axons from both ways’ can be eliminated. Now the question becomes whether the optic tract contains information from one eye or two. The answer is two. Information from the nasal hemiretina (medial half of the retina) of the left eye cross the optic chiasm and enters the right optic tract. The right optic tract is also made up of nerve fibers originating from the temporal hemiretina of the right eye. In that way, the right side of the brain processes information from the left side of the visual world.

The question tells us we see a consensual but not a direct response. We can therefore conclude that the optic nerve of the left eye is intact because the optic nerve of the right eye is not involved in the response. The response involves information going down the left optic nerve to the pretectal area. From the pretectal area, neurons project bilaterally to the Edinger-Westphal nucleus. Axons from neurons in the nucleus innervate the ciliary ganglion. We see a consensual response in the right eye. We can therefore conclude that the bilateral projection and the efferent pathway to the right eye arc unharmed, and that there is some problem with the efferent pathway to the left eye. We are not seeing a constriction of the left pupil in response to the light being directly shone into the left eye. Conclusion: The left optic nerve is intact, but the efferent pathway of the left eye is somewhere and somehow damaged.