Which of the following is least absorbable in renal tubules?

Creatinine is an end product of creatine metabolism and is expoed into our blood continuously by skeletal muscle. The rate is propoional to skeletal muscle mass, and to the extent that muscle mass is constant in a given individual, the creatinine production is constant. Creatinine is freely filtered and not reabsorbed. A small amount, however, is secreted by the proximal tubule. Therefore, the creatinine appearing in the urine represents a filtered component (mostly) and a much smaller secreted component. Because of the secretion, creatinine clearance is slightly higher than the GFR, normally by about 10% to 20%. Ref: Eaton D.C., Pooler J.P. (2013). Chapter 3. Clearance. In D.C. Eaton, J.P. Pooler (Eds), Vander's Renal Physiology, 8e.