The answer is medulla. Milady states that although mature male beard hair contains a medulla, this innermost layer of the hair may be absent in very fine and naturally blond hair.
That's a notable exception to the general three-layer structure (cuticle, cortex, medulla) the book teaches — the medulla is composed of round cells and sits at the very center of the strand, but it's genuinely optional in some hair types, unlike the cuticle and cortex, which are always present.
Milady Standard Barbering, 6th ed., Ch. 10 (Properties and Disorders of the Hair and Scalp), p. 250: "Although mature male beard hair contains a medulla, this layer of the hair may be absent in very fine and naturally blond hair."
This is 1 of the real Hair Science questions in FadeJunkie's Milady-sourced 300-question bank. See the Hair Science practice questions page for a fuller sample, or try the Hair Science practice quiz and flashcards for free. A one-time $15 lifetime pass unlocks the complete question bank across all 13 tested topics.
More real Hair Science questions with sourced answers: What Usually Causes High-Porosity Hair, What Is Terminal Hair, What Is Hair Called When It Feels Smooth With a Compact Cuticle, Which Growth Phase Forms a Rounded Club at the Root, and 28 more on the Hair Science practice questions page.
Browse every topic on the Practice Question Hub, or see the Texas Barber State Board Exam Guide for the full exam process — required hours, PSI scheduling, and fees.