The combining form lact/o means which of the following?

• A. Potassium
• B. Milk
• C. Larynx (voice box)
• D. Stone, calculus

Answer: (B)

The concept being tested is identifying a medical combining form and its meaning. The combining form lact/o comes from Latin for milk, so it is used in terms related to milk and milk products. This is why lact/o appears in words like lactose (milk sugar), lactation (the act of producing milk), and lactiferous (bearing milk). The other roots point to entirely different ideas—potassium uses a root related to kal/i, the larynx is laryng/o, and stone or calculus comes from lith/o or calc-/calci-. So lact/o specifically means milk.