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Using undergraduate calculus, we give a direct elementary proof of a sharp Markov-type inequality $\parallel p^{\text{'}}{\parallel }_{[-1,1]}\le \frac{1}{2}{\parallel p\parallel }_{[-1,1]}$ for a constrained polynomial $p$ of degree at most $n$, initially claimed by P. Erdős, which is different from the one in the paper of T. Erdélyi (2015). Whereafter, we give the situations on which the equality holds. On the basis of this inequality, we study the monotone polynomial which has only real zeros all but one outside of the interval $(-1,1)$ and establish a new asymptotically sharp inequality.