This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
Which one of the following about enzymes is true? 
Select and explain. 
A. High Km-High affinity, B. Low Km-High affinity, C. Non-competitive inhibition- Decreases Km, D. Non-competitive inhibition – Increased Km
B: Low Km-High affinity
Answer: B: Low Km-High affinity 
Explanation: (Refer: DM Vasudevan, Textbook of Biochemistry, 7th edition, pg no: 59 - 60)
The Michaelis constant (km)

Km provides a measure of the affinity of the enzyme for its substrate.
It is equal to substrate concentration at which reaction velocity is equal to ½ Vmax.
Km = substrate concentration when reaction velocity is % Vmax.
Km is constant for any given enzyme.
Km is independent of substrate or enzyme concentration.
The higher is KM, the higher is the concentration of the substrate required to reach ½ Vmax.
That is: at a lower affinity, a higher concentration of substrate is required to reach ½  Vmax.
↓ Km = ↑ affinity.