Question at position 1 John accidentally steps on a sea urchin while wading in the surf at the beach. The sea urchin spine penetrates all the way through the dermis on the sole of his foot. Through how many layers of John's epidermis did the sea urchin spine penetrate?456231Single choice
A
4
B
5
C
6
D
2
E
3
F
1

Question

Question at position 1 John accidentally steps on a sea urchin while wading in the surf at the beach. The sea urchin spine penetrates all the way through the dermis on the sole of his foot. Through how many layers of John's epidermis did the sea urchin spine penetrate?456231Single choice

A

4

B

5

C

6

D

2

E

3

F

1

BlackTom AI · Accepted Answer

The sole of the foot is thick skin and therefore contains five distinct epidermal strata: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.

1) Relevant knowledge points and concepts
- The epidermis is organized into layers (strata) from deep to superficial. In thin skin (most of the body) there are four layers because stratum lucidum is absent; in thick skin (palms and soles) there are five layers because stratum lucidum is present. The dermis lies deep to the epidermis.

2) Step-by-step reasoning
- The problem states the sea urchin spine penetrates all the way through the dermis on the sole of the foot. Since the epidermis overlies the dermis, the spine must have passed through every layer of the epidermis first.
- Because the sole is thick skin, we must count five epidermal layers.

3) Intermediate steps / explicit layer list (deep → superficial)
- Stratum basale (germinativum) — basal proliferative layer
- Stratum spinosum — keratinocytes with desmosomes (“spiny” appearance)
- Stratum granulosum — keratohyalin granules present
- Stratum lucidum — clear layer present only in thick skin (soles/palms)
- Stratum corneum — outermost keratinized dead cell layer

4) Why the correct answer is correct
- The sole is thick skin; therefore the epidermis has five strata. Penetration through the dermis on the sole implies traversal of all five epidermal layers listed above.

5) Why other options are incorrect
- “4” corresponds to thin skin (no stratum lucidum), which does not apply to the sole. “6” is incorrect because there are only five epidermal strata in thick skin. Options “2,” “3,” and “1” undercount the actual number of epidermal layers.