The foot points straight ahead, but the rest of the leg is turned inward 45 degrees. The leg should be identical to the extended one.
If you were going to show off every part of the human body, you wouldn’t do a symmetrical picture, but pictures from all sides. Think of it like mugshots. One in front, one on the side.
Or an early Cubist precursor.
But he only did it with part of one leg? And not the arms at all, or the face? No, if he was trying to be Picasso, he failed.
His body is front on, but from the hips down they begin to turn into a 3/4 view. It looks right to me. I think your expectations are coloured by comic book anatomy, and aren’t seeing what Leonardo was going for.
That seems like the most likely answer. Da Vinci drew exactly what he observed, down to the most minute detail. Even the tiny little penis. 
It must have been freezing in that room.
Rickets was common enough that the model didn’t seem unusually bow-legged. Especially among the lower class that had terrible diets.
Also, the bend really is unnatural. Even for an ultra-curvy woman – nah. I understand that one leg has been rotated, but i dont really think that accounts for the curve!
Da Vinci was trying to show too much in the one drawing.
- the geometrical proportion and symmetry
- profile and front-on aspects of the feet
- anatomical precision
If the mans two middle legs were slightly apart the diagram could accommodate the different views of the feet and avoid obscuring one leg with the other.
I just traveled back in time and asked him* - he said he wanted the feet to touch at the point of symmetry whilst showing one foot front-on and the other in profile. The only way this could be done was putting a bow in the subjects right leg. It looks like he originally was going to put the subjects right leg in profile as well as the left, but changed his mind halfway down and showed the right foot front-on.
Is that little smudge by the foot part of the mistake?
- any answer is just conjecture
Does this statue make it any clearer?
Actually in this view it’s his left leg (viewer’s right) that looks more awkward.
It seems pretty clear to me that you’re looking at an exaggerated soleus muscle.
